THE INTEGUMENT AND DERMAL SKELETON OF SILUROIDEA. By HAMID KHAN BHATTI' ?- tt ~· DQ Zoological Laboratory, Cambridge. c.. CONTENTS ~ 1. HISTORICAL a. Integument • • • • •• • • • • • • • • • 4 b. Dermal Skeleton • • • • • • ••• • • • 7 II. TECHNIQUE AND IVlETHODS • • • • • • ••• • • • 10 III. LIST OF SPECIMENS STUDIED • • • • • • • • • 15 IV. FMAILY LORICARIIDAE • • • • • • ••• • • • 18 A. Sub-Family Plecostominae: 1. Plecostomus GRONOW. ••• • •• • • • 19 a. Integument • • • • •• • • • • • • 19 b. Dermal Skeleton • • • • • • • •• 26 (i) Structure of the Scutes ••• 26 (ii) Structure of the Denticles ••• 33 (iii) Development of the Scutes • • • 39 (iv) Development of the De:nticles • • • 40 (v) Fin Skeleton • • • • • • • • • 47 2. Ancistrus KNER. ••• ••• • • • • • • 49 3. Li202terichthis NOR11. • • • • •• • • • 51 2 Page 4. Xenocara REGN. • • • • • • • • • • •• 53 5. Pseudacanthicus BLKR. • • • • • • • • • 55 B. Sub-Family Hypoptomatinae • • 6. Otocinclus COPE. • • • • • • ••• • • • 60 c. Sub-Family Loricariinae • • 7. Loricaria L. • • • • • • • • • • • • 63 D. Sub-Family Argiinae • . 8. Arges c. & v. • • • • • • • • • • •• 68 v. FAMILY CALLICHTHYIDAE • • • • • • • • • • • • 76 1. Callichth;';lS L. • • • • • • ••• • • • 77 a. Integument • • • • • • • • • • • • 77 b. Dermal Skeleton (i) Structure of' the Scutes · ••• • • • 80 (ii) Structure of' the Denticles • • • 89 (iii) Fin Skeleton • • • • • • ••• 91 2. Hop lo sternum GILL. • • • • • • • •• 94 . . . I 3. Corydoras LACEP. •• • • • ••• • • • 99 4. Development of' the Scutes ••• • • • 108 s. Development of' the Denticles • • • • • • 111 3 Page VI. FAMILY DORADIDAE 1. Deras GTHR. • • • • • • • •• ••• 114 VII. FAMILY BAGRIDAE • • 1. Rita H.B. ••• • •• • •• • • • 118 VIII. DISCUSSION • • • ••• • • • • • • 127 IX. SUMMARY • • • • • • • • • • • • • •• i49 x. LITERATURE • • • • • • • • • • •• • •• lSS' XI. EXPLANATION OF PLATES • • • • • • • •• 17i XII. PLATES • • • • • • • •• • •• • •• I - XXVII. 4 THE INTEGtr~NT 1-..ND DERLtAL , ..-;-,LETOI OF SILUROIDEA HISTORI:J1 a . Integum.en t : - by - Hamid Khan Bhatti, Zoological Laboratory, Cambridge . The various elements of t he epidermis of fishes have been fully investigated by IEYDIG (1851, - 57, - 79, - 95), COLLIKER (1860) , SCHU:.TZE, r: . J .s . (1861), CHULZE , F .E. (1867, - 69) , ".'ffiIGHT (1884), LIST (1885, - 86) , FRITSCH 1886), REID (1894), STUDNICKA (1903, - 06 , - W, - 09) , I:"ULCZYCKI u. NUSRU:M (1905), NUSBJ .. UI{ u . KUL ZYu:CI (1906), NUSB..:m: (190?), 0~ t::::R (19 5) , .:1..·1Lo SKY (190? , - 09 , - 11) , FL.UTHER (190?), NO D UIST (1908), ID . (1911) , and others. Of the naked Silure-idea 7/RIGHT (1884) investigated the structure of .. iurus, V1ULL.~T (1895-96) of ynodontis, s~HULZE, F . E. (186?) of ui urus , FRITSCH { 1886) and LEYDIG (1895) of J'.Ialopterurus electricus , R..~UTHER (190?) of Saccobranchus, and OYJr.ER (1905) studied th club cells of .aruiurus nebulosus and Silurus glani s. The armour clad Siluroide ( "Panzerwelse" ) - Callichthyidae and Loricariidae - however , have so far received very little attention . H.ERT'JIG (1876) in his cl assical work on "The Skin keleton of Fish" makes a brief allusion to the integument of Hypostoma (. Plecostomus ) s consisti ng of "thin epidermis and thi ck cutis , and in the superf'ici 1 layers are numerous cells , purely spherical gland cells as seen in fishes" . filUTHER (1907) first demonstrated the presence of different kinds of gland cells in Callichthyidae and Loricariidae . The epi dermis of iluroidea , like all other fish.es , is gener lly rich in unicellular gl3.Ildular structures such as ( a) mucus cells ~· chleim.zellen" of LEYDIG (1851) , a l so called goblet cells or ".Becherze len" of SCHULZE, F. :E . (186? ) ] ; and (b ) the club cells ( cl v te cells of ~IGHT (1884) or nrolbenzellen" or "Xolben\'"'formige Gebilde" of SCH1JLTZZ , r • .r.s . (1861) ) • In 1884 1. IGHT. pointed out that the "club cells are chiefly developed in these forms where the skin is nak d or the scales are rudimentary (Lota) and no doubt they are en- gaged in the secretion of some substance hich acts as a pro- tecti on in lieu of these" . In 1905 OXNER investigated the form, the distribution , the forraation and the importance of the club cells and included Siluroidea, such as Silurus e,lanis L., .H.miurus nebulosus LES .t!.iUR, nd I~alaopterurus f electricus L.AvEP., in his 11st of fishes hich possess club 6 cells . OXNER (19 5) pointed out that "wi t in the Order Teleostei club cells only occur in Physostomi excepting the f ami l y Salmoni dae in wh i ch these cells are lacking entirely." He regarded club cel l s as unicellular glands having a double funct i on : one of supporti ng and protecti ng the soft epidermis and the other or se cretion , and renarked t hat "their whole protoplasm undergoes a slimy gel tin- like transformation nd the cell s a whole is eventually expelled . " RlUTHER (190?) whi le investigating the skin gl ands of iluroidea pointed out the correlation t h-t exists between the fre uency of club cells and the fonnation of dermal bony scutes . He remarked that " i n Saccobranchus there are numerous club cells , in Callich thys there i s limitation to one layer" . He f ound cells resembling club cells in young Ple costomus , but "in the adult Loricariidae there i s a total absence or such cells" . He observed , therefore , that "all fishes with scales without any relation to thee idermis (cycloid scales ) and those with the skin naked or with reduced skin skeleton (Cyclostom , many Siluroidea, Eel and Lota) have club cells, others do not possess them. lack them. Elasmobranchii hich are clad with skin teeth cipencer , Protopterus (with skin spines) h ve in their skin a greater nwnber of slime cells but no club ' cells . ong ..:1.c nthopterii and Plectognathii , fishes with skin spines, the club cells are never formed". With regard to Siluroidea R.AUTIIER (190?) based his conclusions on the in- 7 vestigations of three members of armour clad Siluro idea - Plecostomus, Loricaria and Callicht ys ( .? Corydoras) . It has therefore been considered necessary to undertak e an investigation of the structure of the integument o f as many members of the Loricariidae and the Callichthyidae as are available so as to study the frequency of the club cells and their correlation witn the dermal skeleton. The s tudy affords an important line of investigation of the phylogen y of armour clad Siluroidea and also provides an additional bas is for the comparison of the relationship that exists between the various sub- families of the Loricariidae and the Callichthy idae . b . Dermal Skeleton The dermal scutes with their articulated denticles of the armour clad Siluoridea ("Panzerwelse") of South America have attracted the attention of various investigato rs, but very few have investigated their mic ost:ructure and their development. Of these still fewer have concerned themselves 1ith ~he study of the microstructure and developmen t of the dermal denticles and the dermal scutes of Callichth yidae. There still , therefore, exists such diversity of op inion ~th regard to their homologies that, as suggested by RR (1919) , "fuller investigation" are necessary. 8 .AGA SI (1833-43) w s the earliest investigator to study the scales of iluroide and struck with the resemblance of their formation to that found in the Ganoids he included them in that Order. He described the scale of Hypostoma plecosto- ~ CHONN., and of Callichthy miles as consisting of "an enamelled outer surface as in Ganoids and of an inferior bony layer" . YITLLI.AMSON 1851) :pointed out the absence of enamel layer of G.A.SSIZ i n Loricaria cataphracta L., and B UDELOT (1873 } added a brief description of the scales of Hypostoma and its dermal denticles . But neither AGASSIZ , nor WILLIAI.:SON nor BaUDELOT noticed the enamel tip of the denticles and it was BE N .... (1873) who first sho d that the :points of the denticles on the scutes eto.ntaculatus CU:V.., have a yellort brownish cover which "in its react ion towards acid is similar to enamel in teeth" . HERTWIG (1876} corroborated the st tement of E:i.Il KE and described the structure of the denticles and the scutes and the fins of Hypostona commersonii and Callichthys longifilis and Callichthys sp. HE. TWIG'S :pioneer work on the skin skeleton of fish is based on the microscopic examination of· the structure of the scales and denticles, and though he hi!!lSelf admits that, in the case of the Siluroidea , he made no observations on the development of the pediment of the denticle and did not study the mode of growth and formation of the scale, yet PO L.cutD (1895) maintains 9 that the skin ossifications of Siluroidea have been "e:r..hauet- ively inves-t;igated" by EERT~a:G. RASE (1911) is the first investigator to point out that HE .T-rrG i n his work on "the skin skeleton of f'ish" has not t all gi ven an account of all details sit is often maintained in literature . All other invest i ators , KNER (1854 ), GOLDI (1884 ), GUNTBER {18?0 ), GOODRICH (1909) , LE GE (1 922 ) and others , have i nvesti gated either the morphological structure of the dermal scutes and the denticles or their relation to the inner skeleton , but have not contributed to the knowledge of the microstructure nor the development of these structures . In 1919 C.ARTE1 while in- vestigating the development of the denticles on the snout of ~:iphius gladius and Gadidae pointed out that "the condition in Pseudacanthicus serr tus c. 0 • v., a Loricariid, is identical with t hat seen in certain of the Gadid e , where the tooth , the pediment and the connecting substance develop on the surface of one dentinal papillatt . mhe study of the structure of the denticles, their de- velopment , their mode of attachuent to the underlying bone, the structure and the growth of the dermal scutes in Loricarii - dae and Callichthyidae has , therefore, been undertaken to determine thPir homologies . II. .u\JHl.'JI' UE .. ~ .JI:TI{ODS tudy of the integwnent has been m de by serial sections of the ventral and lateral skin by embedding in paraffin, and as the material was mostly formalin fixed sections ~ere cut to the thickness of 6 to 10 p.- . The various stains used were the following : (1) Iron haematoxylin counterstained by Vdll Gieson , Rubin in picrate of ammoni , ~aurefuchsin and toluidin blue ; (2) Delafield haematoxylin counterstained by osin , Orange G., aurefuchsin; (3) Unna.'s Orcein ; (4) .lei - gert's Resorci n-Fuchsin ; (5) Thionin , uethylene blue and Toluidin blue for mucus glands and (6) Bielschowsky ' s silver nitrate for sections for nerve endings . The method used for the study of the dermal scales and the dermal denticles has been chiefly the preparation of ground and decalcified sections of the scales and their study as a whole after tre tment with 10% caustic pot sh . The de- calcifying solutions recommended by l IC P.S H (1893) .TEir-irINS (1921) and Kn GSBURY and J IkN r r:; T (1927) were used but as the material was not fresh the best results were obtained 11th z:::IGLE 's solution of 5 % sulphurous acid. ~fter decalcifica- tion the material was washed in water for twenty-four hours and dehydrated slowly . The s ctions were cut either by em.beddin in p raffin or by double embedding of paraffin and celloidin. 11 The stains used were iron~ em toxylin and el field haem.a- toxylin counterst ined with Eosin , urefuchsin, Van Giesen , Rubin in picrate of ammoni and Lallory's triple stain. Preparation of ground sections Ordinary methods of grinding by rubbing down the object with pumice stone with finger or by placing the object on cork or a piece of soft wood and then grinding it on a fine corborundum stone give satisfactory results with large pieces of bone or teeth , o~t coulu not bee ployed to grind transverse or longitudinal sections of the scales varying in thickness f r om 1 mm. to 3 mm . G. von KOCB. ' s copal method (1878) of embedding the object in a solution of copal , cementing it to a slide and then grinding it talces quite a long time. ;r lSTONE-:;:. •. 1.1.VIS an VOSL."R ' s balsam method (1887) by embedding the I'laterial in benzole and then in benzole balsa.Li and of ·.7EIL as recommended by l r =:RY ( 1891 by embedding in chloroform , ether and can da bQlsam and then grinding it, t ke several days. The method employed for grinding such minute objects as the scales and denticles of Siluroidea is a modification of the dry canada balsaL1 method recommended by HOGG (1911) by which the object is embedded in dry canada balsam. By this method ·it is possible to obtain very thin sections in a comparatively short time. HOGG's method has been modified so as to enable one to 11 The stains used were iron.hem toxylin and el field hem.a- toxylin counterst ined with osin, Saurefuchsin , Van Giesen , Rubin in pier te of arnmoni· and ~allory's triple stain. Preparation of ground sections Ordinary methods of grinding by rubbing down t he object with pumice stone with finger or by lacing the object on cork or a piece of soft wood and then grinding i t on a fine carborundum stone give satisfactory results with large pieces of bone or teeth , but could not be employed to grind t ransverse or longitudinal sections of the scales varying in thickness from 1 mm. to 3 mm . G. von KOC:H ' s copal method (18?8) of embeddi ng the object in a solution of copal , cementing it to l -- a slide and then grinding it takes qu ite a long time . J" L.-TONE-LVIS an VOSI.!ER ' s balsam method (188?) by embedding the material in benzole and then in benzole balsam and of ~1CIL as recommended by nm ::RY ( 1891) by embeddi ng in chloroform , ether and canada balsam and then grinding it, take several days. The method employed for grinding such minute objects as the scales and denticles of Siluroidea is a modification of the dry canada balsam method recommended by HOGG (1911) by which the object is embedded in dry canada balsam. By this method it is possible to obtain very thin sections in a comparatively short time. HOGG's method has been modified so as to enable one to 12 polish nd stain the sections without taking them off the slide. This is a great advantage since the denticles are sometimes so loosely attached to the scales that if handled roughly they fall off all too easily . Cut the scale with a p ir of scissors or fine saw trans- versely or longitudinally , as desired ,- into such small pieces as can be held by forceps . Take a piece of solid canada balsam on a glass slide and heat it gently so as just to melt it . Let it cool a little and then embed the section in it i'n such a manner that the cut surface of the section touches the slide . The alsam.hardens very quickly . The section is then rubbed down on a hone witil quite smooth and polished on a dry hone. The slide is then placed ither in methylated apiri t as recommended by H GG or in 90 io alcohol so as to loosen the section. In a very short time the section is sep- ar ted off , dried and then embedded again in dry canada balsam on e. glass slide in such m nner that the ground and polished surface is touching the slide. It is then ground down on a grindstone or rough carborundum and then on a hone to the requisite thinness and then polished on a dry hone. By this method it has been possible to grind sections of rock fossils almost to transparency. Transverse sections of the derm 1 denticles and of very small scales which are less than a milli - meter in diameter have qlsa been satisfactorily ground dovm and 13 for this purpose Hogg's m~thod of embedding the minute objects in dry balsam on a piece of mica was employed . he object is cemented to a thin piece of mica and then attached to a glass slide by the same means and ground and polished on its upper surface . The slide is then heated just sufficien~ly to separate the mica piece together with tbe object embedded in it . The mica piece is then cemented to another slide ith the ground surface of tbe object t owards the slide . The mica piece breaks when further grinding is don and the object is now ground down to transparency and polished. lhile grinding plenty of water is to be used to wash away t e bone and the canada balsam, and also to prevent the heat by friction which otherwise would soften t e balsam and loosen the section. The section when ground and still sticking to t he slide is washed in w ter, stained and dehydrated. The slide with the ground section is then put in slightly warm xylol to dissolve the hard b sam. The section till re.main sticking to the slide if it is not kept too long in xylol. s soon as the balsam around the section is dissolved remove the slide and put a drop of fresh canada balsam over the section and cover it vath a coverslip. If it is desired to mount the section dry, it is not cleared in xylol, and instead the slide is heated gently so as to melt the balsam around the section and a coverslip is placed over it as quickly as possible. The air is retained in the lacunae and canaliculi, which appear as black spots and 14 lines. The ground section can be stained ith picro -indigo carmine, bora cannine , thionin, acid fuchsin and eosin. Del field haematoxylin is not as tisfactory stain as owing to the presence of calcareous matter it leaves a deposit which is easily rubbed off when the sections are washed in water. The slide with the section is immersed in the stain for the desired length of time and then washed. Piere-indigo c rmine stains the dentine and bone blue and the rest or the structure greenish blue . The lacun e, their canaliculi and the dentine tubes appear opaque and bl ck as the balsam cb es not penetr te into them and leaves them full of air. Prep ration of combined ground and decalcified sections In order to study the structure or part decalcified and part not decalcified in a sin le section HANJ.ZAWA ' s method (1917} w s adopted with the modification neoess ry for the dry canada b sem method . The transverse or the longitudinal section of the scale is ground byte dry canada balsam method, washed, stained wit.h picro indigo carmine or fuchsin and then dried. Half of the section is then smoothly covered over with melte sot paraffin by means of a sialpe. en the paraffin solidifies the slide is put in a decalcifying solution (hydro- chloric acid in water or in 70 fa .1lcohol) for 5 to 10 minutes , so that the half of the section not covered by the paraffin is r decalcified . · The slide is then washed in water to remove the traces of t he ac i d, and the decalcified part of the section is then stained with thionin or Del field haematoxylin, ..ti'ter dehydration t he slide is immersed in warm xylol so as to melt and absorb the paraffin from the non-decalcified part . As soon as the section is clear it is mounted in fresh canada balsam. The difference in the a~pearance of the structure of the two parts is remarkable . The part which has been covered with par af fin and thus protected from the action of the acid shows bone lacunae with canaliculi and the dentine tubes far bette r and more distinctly than the decalcified part. The structure of the integument, t he dermal denticles and the dermal scales of the following fishes have been studied: F 1J1ILY LORICARIIDAE Sub-Family Plecostominae : Genus Plecostomus GRONOW. Plecostomus plecostomus L. ; Plecostonus comm.ersonii c. &_V . ; Plecostomus sp. Genus Ancistrus KNER. ncistrus multiradi tus II.'.ili'CK. ; .Ancistrus setosus BLGR. Genus Lipopterichthys NO • Lipopter i cht hys carrioni NOBM. :.J Genus Xenocara REGN. Xenocara hoplogenys GTER. Genus Pseudacanthicus BLL"'"R~ Pseudacanthicus serratus C. & V. Sub-Family Hypoptoruatinae . Genus Otocinclus COP 1 • 16 Otocinclus vittatus RGN., Otocinclus nigrioauda BLGR. Sub-Family Loricariinae . Genus Loricaria L. Loricaria typus BLKR. Sub- Fa,mily ..\rgiin e. Genus rges C. · V . ..a.rges suentheri B GR . ; rges brachycephalus GTHRo Genus Callichthys L. Callichthys callicthys L. ; Callichthy pectoralis. Genus Hoplosternum GIII.. Hoplosternum littorale HANi • Genu Corydoras orydoras eneus GI • ; Corydoras elegans ~IND. ; Corydoras triline tus 001£; Corydoras kronei RI:ar::IA Corydor a psleatus JE rr. ; ~orydoras g ssizii SY£IlID. • , 17 FAMILY DORADIDAE Deras weddellii C.ASTELN.; Deras dorsalis C. & v. FAMILY BAGRIDAE Rita rita H.B. For the supply of the material I am indebted to Mr. J. R. NORMAff, of the British Museum, Dr. ERNEST AHL, Zoologische Museum, Berlin , Dr. G.S. C,A.RTER of Cambridge. Professor GRAHAM KERR of Glasgow, Mr. M.A. HUSAIN and Dr. G. MATTHAI of the Punjab University. I am extremely grateful to my teacher mr. c. FORSTER - COOPER for suggesting the problem. The whole work has been carried out under his supervision and my best thanks are due to him for his constant encouragement, keen interest and for his assistance in completing the work. I am thankful to Mr. J. THORNTON CARTER of London University for reading part of my MS. and encouraging me to complete the investigations. I am indebted to Professor J. STANLEY GARDINER for affording me every facility to work in the Zoological Laboratory at Cambridge. I owe my thanks to Dr. w. LISSMAID,l for the trans- lation of the Literature in German; and to Professor M. RAUTHER for affording me an opportunity to discuss the problem with him at Stuttgart. FAMILY LORICARIIDAE The LORICARIIDAE, a family of the Order Ostariophysi and sub-order Siluroidea, are found only in the rivers of South America. REGAN (1903) divides the family into five sub- families: Plecostominae, Hypoptomatina~, Loricariinae, Neoplecostominae , and Argiinae. Excepting the degenerate sub-family Argiinae, which are naked, most of the fishes of the family Loricariinae are protected by an armour of bony scutes on their body. The abdomen is either naked or has patches of granular scales or is covered with bony plates ei- ther in longitudinal series or irregularly arranged. The body scutes as well as the ventral scutes are beset with little spines or denticles. REGAN (1903) has pointed out that it is often the case that these denticles "are stronger in the males than in the females''• Thus for instance in Plecostomus spinosissimus STDR., and Plecostomus festae BLGR., the males have the head and the body covered with quite str6ng ·jspines. In Ancistrus setosus BLGR., and its allies and in many species of Oxyloricaria BLKR., Farlowella EIGM., and Loricaria L., the sides of the head in the male are margined with denticle-like bristles, which are either absent or comparatively shorter in · the female. In Loricaria lanceolata GTHR., and its allies similar bristles occur on the supra-occipital, the nuchal scutes 19 and the spines of the pectoral fins. Sub-Family Plecostominae. Plecostomus GRONOW (Syn. Hypostomus LACEP., Rhinelepis SPIX.) The upper part of the body is protected by bony plates. There are five longitudinal series of scutes on each side of 1 the caudal peduncle for its whole length. The abdomen in the adult is either naked or is covered with small granular scales. The operculum and the interoperculum are incapable of independent movement and are with or without bristles. The lower transverse portions of coracoids and clavicles are not exposed (REGAN, 1903). Integwnent. The structure of the integument of the adult Plecostomus (Hypostoma) has been described by HERTWIG (1876) and of the young by RAUTHER (1907). RAUTHER (1907) investigated the structure of three specimens of Plecostomus commersonii c. & V. measuring 7 cm., 3.5 cm., and 2.2 cm. and found the following differences in these stages. Specimens measuring 7 cm. had beaker-shaped mucus cells (''Becherzellen") and granulated cells ("Kornerdrtisen"), whereas club cells (1) Caudal peduncle refers to the tail region of the animal posterior to the insertion of the pelvic fins. Length of the animal wherever given is measured upto the base of the caudal fin. ( "Kolbenzellen'') were lacking, in the specimens measuring 3. 5 cm. and 2.2 cm. club cells were present and in the latter specimen they opened on the surface of the epidermis while the granulated cells were absent. RAUTHER observed that owing to the difference in the occurrence of these cells "in young stages the developing gland cells take the shape of •Leydig 1 cells , whereas in the older stages there develop:.6 the granulated cells''. He, however, did not observe the change from one to the other, In Plecostomus sp., measuring 18 mm., obtained from the British Museum, the epidermis consists of: (1) three layers of epidermal cells, the basal, the middle and the superficial, The basal layer has large oblong cells each with its round base resting on the basement membrane and its apex directed towards the surface of the epidermis, with a large nucleus lying in the middle (FIG. 1, epb). In the .. middle and the superficial layers the cells are sm3.ller than those of the basal layer and are almost wholly occupied by large nucleus; (2) in be- tween the cells of the middle and the superficial layers are beakershaped mucus cells ("Becherzellen") with the nucleus lying towards the peripheral wall at the base. The mucus (1) Prof. RAUTHER calls the club cells 11 Leydif cells 11 but in his letter to the author he had advised him to use the name club cells in preference to Leydig cells, which term, he suggests, should better be reserved for peculiar big cells in the epidermis of Urodele larvae. 20 ("Kolbenzellen'1 ) were lacking, in the specimens measuring 3.5 cm. and 2.2 cm. club cells were present and in the latter specimen they opened on the surface of the epidermis while the granulated cells were absent. RAUTHER observed that owing to the difference in the occurrence of these cells 11 in young stages the developing gland cells take the shape of •Leydig 1 cells , whereas in the older stages there develop:. ... the granulated cells". He, however, did not observe the change from one to the other~ In Plecostomus sp., measuring 18 mm., obtained from the British Museum, the epidermis consists of: (1) three layers of epidermal cells, the basal, the middle and the superficial, The basal layer has large oblong cells each with its round base resting on the basement membrane and its apex directed towards the surface of the epidermis, with a large nucleus lying in the middle (FIG. 1, epb) • In the .. middle and the superf±cial layers the cells are smaller than those of the basal layer and are almost wholly occupied by large nucleus; (2) in be- tween the cells of the middle and the superficial layers are beaker shaped mucus cells ( ''Becherzellen'') with the nucleus lying towards the peripheral wall at the base. The mucus (1) Prof. RAUTHER calls the club cells "Leydig' cells'' but in his letter to the author he had advised him to use the name club cells in preference to Leydig cells, which term, he suggests, should better be reserved for peculiar big cells in the epidermis of Urodele larvae. cells take a violet stain with methylene blue, toluidin blue and deep violet with Delafield haematoxylin. These cells develop from the basal layer and open to the exterior by a short neck (FIG. 1, !!!£,); (3) towards the ventro-lateral bor- der of the animal near the anus the epidermis contains a few large round cells which occupy almost the whole thickness of the epidermis (FIG. 2, g£), and are filled with granular acidophil contents (g,£) which stain red with acid fmchsin and eosin, A large nucleus(!!£.) lies towards the base. These cells open to the exterior by a short neck. RAUTHER (1907) describes these cells as "Leydig cells" or club cells corres- ponding to the club cells found in other Physostomi. But the position of their nucleus and their opefng to the exterior are characters ·such as are not f ound in typical club cells which in Siluroidea have their nucleus invariably in the middle and do not open to the exterior. These large round cells be- long to the category of the so called granulated cells or .. . . ... ,.., 11 Kornerdrusen 11 .or 11 Sackform1ge Serose Drusen zellen" as des- cribed by STUDNICKA (1906 a) in Lepadogaster, by KWIETNIEWSKI (1905) in Selachii, by BYKOWSKI and NUSBAUM (1905) in Fier- asfer, by RAUTHER (1907) in adult Plecostomus commersonii and in Callichthys punctatus (? Corydoras melanistis RGN.), and by HASE (1911) in Cyclopterus lumpus, and do not form a transitional stage from club cells to granulated cells as 22 assumed by RAUI'HER (1907). The term 1'Kornerdrusen 11 applied by RAUTHER (1907) to these cells is not an appropriate one as their contents are not always granulated. It is therefore proposed to apply the name sac-cells as an abbreviation of the term "Sackformige Serose Drusen11 introduced by STUDNICKA (1906 a); (4), groups of two or more cutaneous sensory cells (FIG. 3, nh) similar to those described by RAUTHER (1907), WRISHT (1884), LEYDIG (1879,-95) and others. WRIGHT (1884) in Amiurus refers them as 11Nerve-hillocks 11 corresponding to "Nerven-Hugel1' of MERKEL (1880). The cells are large, oval in shape with acidophil granular contents (FIG. 3, ~) with a large nucleus, have a base of small epidermal cells (§.9.) and are situated in a cavity which~ opens to the exterior by an aperture. They rest at the same level as the basal layer of epidermis on the cerium which does not form any papillae or projections. To their bases join small nerve bundles (g). RAUTHER (1907) describes cilia or cirri on their free extremity, but in the specimen under investiga- tion the cilia are not preserved. RAUTHER (1907) recognises them as free accessory lateral sensory organs as have been described by SCHULZE, F.E. (1867) in Gobius, and by LEYDIG (1851), SOLG.AR (1878) and others in Gasterosteus, Esox, etc.; (5) wandwring connective tissue cells, which are small round cells (FIGS. 2& 3, fb) with deeply s~ained nucleus and colourless assumed by RAUI'HER (1907). The term ''Kornerdrusen'' applied by RAUTHER (1907) to these cells is not an appropriate one as their contents are not always granulated . It is therefore proposed to apply the name sac-cells as an abbreviation of the term 11 Sackformige Serose Drusen'' introduced by STUDNICKA (1906 a); (4), groups of two or more cutaneous sensory cells (FIG. 3, nh) similar to those described by RAUTHER (1907), WRISHT (1884), LEYDIG (1879,-95) and others. 'WRIGHT (1884) in Amiurus refers them as ''Nerve-hillocks 11 corresponding to "Nerven-Hugel" of MERKEL (1880). The cells are large, oval ih shape with acidophil granular contents (FIG. 3, ~) with a large nucleus, have a base of small epidermal cells (sc) and are situated in a cavity which.. opens to the exterior by an aperture. They rest at the same level as the basal layer of epidermis on the cerium which does not form any papillae or projections. To their bases join small nerve bundles (g). RAUTHER (1907) describes cilia or cirri on their free extremity, but in the specimen under investiga- tion the cilia are not preserved. RAUTHER (1907) recognises them as free accessory lateral sensory organs as have been described by SCHULZE, F.E. (1867) in Gobius, and by LEYDIG (1851), SOLGAR (1878) and others in Gasterosteus, Esox, etc.; (5) wandwring connective tissue cells, which are small round cells (FIGS.~& 3, :f'b) with deeply s~ained nucleus and colourless 23 plasma in the lower layer of the epidermis. They correspond to "Wanderzellen of LIST (1885), fibroblasts of REID (1894) or Lymphocytes of FRITSCH (1886) and others. In adult Plecostomus (Hypostoma) HERTWIG (1876) distin- guished only two kinds of cells: i.e. "epidermal cells, their basal layer forming a single layer of thin high cylindrical cells, and slime sells, spherical in shape." RAUTHER (1907) in Plecostomus co~Jnersonii,7 mm. long described the granula - ted cells, the slime cells and the sensory cells. Present investigations show that in Plecostomus plecostomus L., 150 mm. long and Plecostomus comrnersonii c. & v. 146 mm. in length the granulated cells are lacking. The epidermis (FIG. 5) consists fi r stly, of several layers of epidermal cells, the basal layer having cylindrical cells (FIG. 5, epb), while the cells in the middle layers are oblong or polygonal in shape; secondly, the mucus cells, which in P. plecostomus have a broad base and a short neck with the nucleus lying at the base (FIG. s, ~), and -in P. comn1ersonii on the ventral skin they are more or less pear shaped with a comparatively long neck which opens to the exterior. Their contents are of faint network and frequently the cells are empty; thirdly; the cutaneous sense organs, which are of two kinds: (i) nerve-hillocks (FIG. 6, nh), similar to those seen in young Plecostomus and (ii) pear shaped groups of six or more sensory cells situated on a papilla of cerium 24 which projects into the epidermis. The cells are cylindri- cal with elongated necks, sometime projecting slightly beyond l the level of the epidermis (FIG. 6, nb ). The nuclei are confined to the base. They are surrounded by a mantle of thin and flattened epithelial cells and have small nerve bundles join- ing to them. Similar sense organs have been described by LEYDIG (18?9), SOLGAR (18?8) and others in Esox, Gasterosteus, ~' Amblyoffrs etc. and by WRIGHT (1884) in Amiurus, where he calls them as "end-buds" as corresponding to the "End-Knospen" of MERKEL (1880); fourthly, fibroblasts or wandering connec- tive tissue cells are present in large numbers in the lower layers (FIG. 5, fb). A distinct limit between the epidermis and the corium is formed by a basement membrane (FIG. 5, .£!!!) consisting of longi- tudinal connective tissue fibres with or without connective tissue cells. The cerium consists of two layers, an outer and an inner. The outer layer (FIG. 2?, .£2.) is not easily dis- tinguishable from the inner excepting in those parts where scutes have developed, where it consists of loose ·rebiCular connective tissue fibres. Black pigment cells (FIG. 31, _££) lie immediately beneath the basement membrane. They are, (1) The section cuts the end-bud obliquely, therefore, the necks of the sensory oells are not seen projecting beyond the level of the epidermis. 25 however, lacking on the ventral skin. In between the fibres are a few blood vessels (FIG. 31, .kY,) and scattered connective tissue cells. At places where the sensory buds are situated the outer layer of the cerium projects into the epidermis to bear them (FIG. 6, nb), The inner layer (FIG. 29, £1) con- sists of thin connective tissue fibres collected into bundles which run horizontally (FIG. 29, .£!!), and are traversed by a few vertical fibres (FIG. 29, .Q.Y), which pass through the successive layers and thus hold the unconnected bundles to- gether. The horizontal bundles form two sets, one running parallel to the longitudinal axis and the other to the trans- verse axis of the skin, are not woven together and have no other support than the vertical fibres. They lie parallel but diverge at places to form cavities. Very few elastic fibres are placed between the lamellated bundles. There are a few blood vessels and a few connective tissue cells, excepting at places where the scutes are developing or the denticles are being formed when the connective tissue cells are found in abundance (FIG. 31, ..2!£.). A s imiler structure of the cerium has been described by NICKERSON (1893) in Lepidosteus. 26 Dermal Skeleton Structure of the scutes ----------------------- VENTRAL SCUTES. In Plecostomus sp. 18 mm in length the ventral skin is free of any ossification. In plecostomus plecostomus, 50 mm long, ventral scutes have appeared but lie scattered in the skin without any regular arrangement. Anteriorly the scutes lie far apart from each other and each is rectangular in shape and carries one to four denticles. Denticles are seen in various stages of development. Posteriorly, in front of the anus, the scutes in the middle of the ventral skin lie close to each other and are either rectangular or polygonal in shape. Towards the sides and just in front of the anus they are again scattered and lie far apart from each other, In Plecostomus plecostomus, measuring 65 mm., the scutes on the ventral skin are well developed and form a mosaic-like pattern in the middle region, with 2 to 8 denticles on each. Anteriorly, however, they lie scattered and are far apart from one another and bear one to three denticles. The ventral scutes, therefore, make their first appearance in the posterior region anterior to the anus and gradually spread over almost the whole of the ventral surface. 1 2'7 The arrangement of the ventral scutes in adult Hypostoma commersonii GUNTH., ( = Plecostomus co.mmersonii c. & v.), has been described by HERTWIG (1876). There is not much differ- ence in the arrangement of the ventral scutes on Plecostomus plecostomus L., measuring 150 mm. and P. commersonii 146 mm , long (FIG. 8, .!£!.). The ventral plates, carrying one, two or more denticles, lie in the corium forming the boundary between the outer and the inner layers (FIG. 11, .!£!.). Their shape varies and they may be rectangular, quadrilateral or triangu- lar. Near the mouth, anus and the base of the pectoral and pelvic fins they lie at some distance from one another. To- wards the middle of the ventral surface the size of the plate increases and they fit with their edges like the tiles of a roof (FIG. 7, .!,!). The area immediately around the anus and around the mouth is free of ossification. Posteriorly between the anal fin and the anus there are two or three small plate- lets with two to four denticles and one large plate at the base of the anal fin. Laterally the ventral scutes (FIG. 11, ~) are found lying over the ventral border of the large lateral acute (J2!!) but separated from it by connective tissue fibres (FIG. 11). The ventral plates at the posterior part of the abdomen lying near the lateral scutes slightly overlap one another, thus producing two areas on the surface of the plate, the anterior smooth area has no denticles and is covered over by the posterior border of the preceding acute, and a posterior area, which bears the denticles. The ventral scute (FIGS 11 and 29, vs) is mainly composed of the pediments of the denticles with a basal layer of iso- pedine which, in the young, has one to two and, in the adult, four or more lamellae of bony tissue containing bone corpuscles with branching canaliculi. The lamellae of isopedine at the margins merge into connective tissue bundles of the corium (FIGS. 11 and 29, _tl) and vertical f'ibres of the corium run in the substance of the acute, {FIG. 29 _!:!and~). The middle portion in small scutes is solely composed of the cavities of the pediment into which blood vessels enter directly from the lower surface of the scute. In large scutes Haversian canals are present which communicate with the cavities of the pediments. Sometimes the Haversian canals run through the entire thickness of the scute and open on its surface, the blood vessels passing out of the acute into the outer layer of the cerium. BODY SCUTES In Plecostomus sp. measuring 18 mm the scutes have developed at the peduncle, overlap each other, and bear one or two rows of two to four denticles. Altogether there are six longitudinal rows of denticles on the peduncle. Transversely the scutes have not yet extended so far as to overlap one 29 another. Towards the anterior end of the body the scutes are not fully developed and do not overlap each other either longi- tudinally or transversely. There are only three longitudinal rows of denticles with one to three denticles in each row on each acute. CUVIER and VALENCIENNES (1828-49), EERTWIG {18?6), REGAN (1903), and LEEGE: (1922) have described the form and arrange- ment of the body soutes in adult Plecostominae. The body is covered with bony plates arranged in five longitudinal rows (FIG. 10) on each side of the caudal peduncle for its whole length. From the insertion of the pelvic fins to the cranium the number of rows diminishes to four, the lowest row begins at the ventral fins where the scutes are spatulate. On the sides of the peduncle the scutes are more or less lozenge- shaped, become smaller and smaller towards the tail and over- lap each other like cycloid scales. At the base of the caudal fin is a number of horizontally placed lancet-like shields. • Towards the middle of the body they lie obliquely, are rhomboid with a concave posterior and convex anterior border and taper to narrow ends dorsally and ventrally (FIG. 10). The lateral line canal pierces through the scutes of the third row (FIG. 10, On the dorsal surface the plates of both sides are separated from each other at the insertion of the first and the second dorsal fins, but in front and behind the fins, are joined 30 together by distinct sutures, as do the ventral scutes behind the anal fin • . The scutes on the body are so arranged as to overlap at the margins and the posterior border of each covers the anterior border of the other. They are also joined to- gether by skin folds in such a manner as to permit free move- ment of the body. The upper surface of the scute is thus divided into two areas just as in the ventral scute already described, (FIG. 9 bsa and bsp). The scutes are mostly carinate, that is, the posterior toothed area is raised in the middle into a keel-like ridge. The scutes carry denticles in rows which diverge antero-posteriorly and those on the carinate part (FIGS. 9 and 10, ~) and the posterior border (FIG. 9, ~) are larger and stouter than those on the remaining part of the scute. The body scute is not of uniform thickness, but is thicker at its anterior border (FIG. 12, bsa) which lies embedded in the inner layer of the corium and tapers towards the posterior border (FIG. 12, bsp) where the thickness of the scute is almost entirely composed of the large pediments of the denticles and two or more lamellae of isopedine. Thus in this point the posterior denticle bearing part of the scute closely resembles the ventral scute. The space between the posterior border of a scute and the thick anterior border of the succeeding one is filled in by the connective tissue fibres (FIG. 12, -2!) which merge into the bony substance of the acute itself. By the 31 extensibility of these connective tissue fibres movement of the body scutes takes place without hindrance and owing to their position such bundles have been named the inter-scale ribbon (Zwischen Schuppen Band) by HERTWIG (1876). The scute has a hard, tough, bony structure and consists of ; (l) an inferior or basal layer of true osseous tissue (FIG. 14, .!!), composed of lamellated isopedine, containing large bone corpuscles. The lamellae run parallel to the lower border of the acute and towards the base are traversed obliquely by fine uncalcified connective tissue fibres, which are not distinctly visible in decalcified sections, but are seen as minute clear spaces in ground sections (FIG. 14, lpd). These structures were described as "lepidine tubules" by WILLIAMSON (1849), who describes in Lepidosteus and Lorioaria cataphracta "an extensive development of minute much-branched tubes which radiate from the outer surface of the inner por- tion of the scale, penetrating the lamellae in oblique direc- tion, verging as they do so from the outer border towards the centre of the scale. Though not the homologue of dentine, they appear to fulfil a similar function in the scales to that which the dentine tubes do in the teeth". Their function he considered to be the "general nutrition of the scale". In Lepidosteus, KI.AA.TSCH (1890) and NICKERSON (1893) have already shown that lepidine tubules of WILLIAMSON are due to the 32 presence of uncalcified connective tissue fibres. The bone corpuscles (FIG. 13, bo) are oval, round or quadrate with numerous fine ramifying canaliculi (boo) which intertwine with those of the neighbouring corpuscles. Their shape and form in section va-ry with its direction. In de- calcified sections they appear as small oval bodies. They are best seen in ground sections, and especially when cut in the direction of their long axis. Their oanaliculi show a tendency to spread out parallel to the lamellae of isopedine within which they have become enclosed, so that the texture of the scute seems to consist of regular lamellae of isopedine alternating with layers of bone corpuscles and their ramifying canaliculi, (FIG. 15, ,.22); (2) an upper layer (FIG. 14, bsp and hsa) formed by the pediments of the denticles on the posterior part of the acute and by bony lamellae on the anterior part. In ground sections (FIG. 14, J:.!) the lamellae of basal layer appear to turn upwards at the anterior border and to be continuous with the lamellae of the upper layer. But as seen in decalcified sections (FIG. 12, J.1!) lamellae of the basal layer actually merge into the fibres of the corium. The appearance seen in ground sections is due to the characteristic arrangement of bone corpuscles and to the deposition of the calcifying salts in concentric semi-circular lamellae; (3) lying between the two bony layers is the vascular layer con- sisting of Haversian canals, which spread out in irregular V 33 network (FIG. 16, he) around the pediments and receive the canals from the denticles. In this middle layer lie numer- ous large bone corpuscles with numerous branching canaliculi {FIG. 15). The upper surface of the scute is covered over by the outer corium layer and the fibres of this layer are attached to its outer border and penetrate slightly into its substance. The vertical ascending and descending fibres seen in the ventral scutes are absent in the body scutes. Structure of the denticles The denticles on the ventral scute are uniform in their size and dimensions and their number in the adult varies from one to ten on each scute, while on the body scute they increase in size and thickness antero-posteriorly so that on the anter- ior border they are weak and thin and on the posterior border thick and stout (FIG. 18, d) with well-marked yellowish-brown enamel tips (FIG. 14, ~). On the nuchal plate they are uni- form in size and are arranged in regular rows. On all the scutes the denticles are movably articulated to their pediments. On the nuohal plate their mobility, however, is very slight. The denticle is pointed at the tip, broad at the base, which however, is again constricted to form a short prolongation (FIG. 15, .fil?_). Each denticle consists of {i) an enamel layer, {ii) dentine, (111) pulp cavity, (iv) connecting area, and 34 ( v) pediment. The relation of the integument to the denticles and the acute is seen in Fig. 12. The scute lies buried in the cerium in such a manner that its anterior border(~) is embedded in the inner layer of the coriu.m, while its posterior border (bsp) with the denticles rises towards the external layer and forms a boundary between the inner and outer layers of the corium. The tips of the denticles pierce the epidermis, while their bases are covered by a thin layer of epidermis which extends up to the point of junction of the base to the pediment. The pediment and the connecting area are covered not by the epidermis but by the external layer of the coriu.m which does not extend above the point of articulation. HEINCKE (1873) and HERTWIG (1876) pointed out the presence of yellowish brown enamel at the tip of the denticle but none of the previous investigators have described the extent of the enamel covering. If the denticles (FIG. 19) are placed under a coverslip on a slide and 2•/0 hydrochloric acid is added and i ta reaction watched under a low power, it is seen that enarrel extends three fourths of the length of the large denti- cles and one third of the length of the small ones below their tip. It is thick at the tip and in reflected light its yellow- ish-brown colour can easily be detected as spreading over the white surface of the dentine. Drawings by camera lucids gives further proof of its extension over the dentine. The enamel disappears completely under the action of acids. As soon as the action of the acid has started dark concentric rings become visible on the surface of the denticle commencing from its middle and ending near its tip (FIG. 19, ~). No such rings have been described in the denticles of Loricariidae by any of the previous investigators. The rings are either com- plete or incomplete annuli and are markings ranged parallel to the exterior of the dentine and concentric to pulp cavity. As they dis~ppear with the decalcification of enamel they seem to be present in its substance. At the junctipn of the enamel with the dentine,the amelo- dentinal junction (FIG. 22, ~),dentine forms an arch. As the action of the acid advances the e_namel appears to be tubular and fine tubes traverse its margin (FIG. 19, .!!!,) • As the decalcification advances Nasmyth's membrane begins to separate (FIG. 22, .!'!!!!), shrinks and hangs over the tip of the den ticle and is not dissolved by the action of the acid. The yellowish brown colour is situated in the substance of the enamel itself. The other instances of coloured enamel are found aIIX>ng Rodents (TOMES, 1923) in which the "enamel of the front of the large incisors is stained of a deep orange colour which is situated in the enamel itself•, and in Soricidae, a family of Insecti- vorous, where the enamel on the teeth is reddish-brown. The body of the denticle consists of hard tubular dentine. The dentine tubes ramify from the pulp cavity and pierce the substance of the dentine w1 th wavy outline and stop short of 36 its external surface . (FIG. 15, -9!). The tubes do not branch at their extremities but during their course they are joined together by fine branches. In fully deve1oped denticles the dentine tubes extend up to the extremity of the base (FIG. 15, db) and are not lacking in the base as stated by HERTWIG (18?60. The pulp cavity (FIG. 15, pea) is broad at the base and gradually narrows towarms the tip of the denticle where it divides into branches and penetrates into the enamel (FIG. 15, pch.) Its branab.es. can be seen hanging from the end of the dentine when the enamel tip is completely absorbed by the action of the acid (FIG. 21, pch). It contains connective tissue, blood vessels and communicates at its base with the cavity of the pediment. The pediment (FIG.?, pdw} is a circular ring-like eleva- tion raised above the level of the scute, lying in the external layer of the cerium and encloses a cavity into which fits the basal prolongation of the denticle, which is, therefore, socketed (FIG. 15, ~). The broad base of the denticle is attached to the rim of the pediment by connective tissue fibres which appear wavy in outline (FIG. 15, ea}. In addition to this attachment its basal prolongation is further attached to the inner wall of the pediment by connective tissue fibres (FIG. 15, db}. AGASSIZ (1833-43) was the earliest investigator to describe the attach.l:mnt of the dermal denticles to the underlying bone by fibrous tissue in Hy-postoma plecostomus SCHONN., ( = Plecostomus plecostomus L.). WILLIAMSON (1851) investigating the structure of the dentioles of Loricaria cataphracta L., differed from AGASSIZ as regards the fixation of the dentiole and pointed out that it is attached to its pediment "by means of capsular expansion of the membrane which covers the surface of the scale". HERTWIG (1876) describes in Hypostoma the ligamentous fixation of the denticle to its pediment and remarks "one sees how the tissue o~ the boundary of the place of articulation suddenly changes into connective tissue fibres which ascend in wave-like outlines and reaching the margin of the rod of the denticle transfer again in the same way to the general substance of the tooth". As HERTWIG {1876) had not investigated the development of the pediment so he could not satisfactorily account for the connective tissue fibres merging into the substance of the tooth. All the previous investigators failed to notice the significance of the double attachment of the dentiole to its pediment: one to its rim and the other to its internal wall. In a freshly developed denticle the double attachment is remarkably clear (FIG. 29, ~), and the denticle is joined not only to the rim of the pediment but the attachment is prolonged down towards the base of the pediment externally as well as internally so that it becomes enclosed within the connective tissue fibres, which, as will be shown when dealing with the development of 38 the denticle, are a direct prolongation of the dentinal papilla and represent neither the "capsular expansion of the membrane which covers the scale" as investigated by WILLIAMSON (1851), nor "the connective tissue fibres of' the boundary o:r the place of articulation" as was believed by HERTWIG (1876), but are actually part and parcel of the substance of the den- ticle itself. The double attachm9nt of the denticles to their pediments obviously restricts their movements, WILLIM!SON (1851) observed that the flanging shoulders of the 'tooth' in Loricaria cataphraota L., as resting on the pediment and its constricted base into the cavity of the pediment produces an arrangement "which closely resembles a ball and socket joint, allowing a considerable degree of motion in every direction". But in reality, as has also been pointed out by HERTWIG (1876) the diameter of the movement depends on the extensibility of the connective tissue fibres and the free space which the rod has in its ~ocket. The denticles in their natural condition lie inclined towards the posterior end of the body on account of the elevated anterior border of their pediments. .Any force in this direction makes them lie flat to the surface, while application of any excessive force in the reverse direc- tion breaks the denticles and very often they are torn off with their pediments from oft the scute. If, however, reasonable force is applied the denticle can be moved slightly 39 in all directions and after removal of the force they resume their normal semi-erect position. Development of the soute No accoW1t of the development of the scute of Loricariidae is available. The scutes whether on the ventral surface or on the body develop independently of the denticles. On the body the scutes appear earlier than on the ventral skin, and com- mence from the tail region and extend towards the anterior part. The scute whether on the body or on the ventral surface, makes its first appearance as a thin sheet of calcareous matter in the midst of the inner layer of the corium (FIDG. 24, ~). It is surrounded on all sides by connective tissue cells (FIG. 31, cto) which ultimately become enclosed within it as bone cells and become distributed through all parts of the scute, (FIG. 32, be). These connective tissue cells or scleroblasts ["Scleroblasten" of KLAATSCH (1890)] are arranged in a more or leas semicircular manner at the anterior border of the ~cute and it is this arrangement and the laying of the calcareous matter in semicircular layers that gives the appear- ance in ground sections of the lamellae of the isopedine turning upwards at the anterior border (FIG. 14, bsa). The acute is not enclosed in scale pocket as are the skin ossifications of normally scaled Telestei. 40 The calcareous matter is not deposited where the blood vessels from below pierce the cerium. These openings persist to form Haversian canals (FIG. 14, .!!£). The scale increase in size and thickness by the calcification of the surrounding corium fibres. The anterior end of the scute is at a lower level than the posterior and as increase in lateral extent takes place the anterior end becomes deeply embedded in the corium and is over-lapped by the end of the preceding acute. The corium fibres extend across from one scute to another and form a flexible connection between the adjacent scutes. The pediments of the denticles, when they develop, join together by bony trabeculae to form an upper layer of the scute. Growth of the scute takes place at its posterior tooth-carrying border. In the skin-fold, coinnecting the posterior border of one scute to the anterior border of another, new denticle buds are formed (FIG. 32, de); and by calcifica- tion of the corium fibres and by the addition of fresh denticles I the scute grows in size. The largest denticles at the posterior border, therefore, are the last to be forned and the thickness of the scute at this place is equivalent to that of the p~d1- ment of the denticle with one or two layers of isopedine. Development of the denticles HERTWIG (18?6) has described the development of the dentioles in HYpostoma and the youngest stages studied by him 41 "showed already a too,th point" with ''dentine cap" covered by enamel. He advances three arguments to show that the denti- cles once formed on the scutes are not permanent formations but have to undergo renewal. "Firstly, that the denticles in younger animal are considerably smaller in number than in the older; secondly, on the ventral plates there are pedestals without denticles which have been destroyed by absorption; thirdly, tooth buds are present in different stages of develop- ment on these platelets in between the fully developed denti- cles". HERTWIG (1876) has described neither the process of absorption nor the development of the pediments, so he did not observe that the absorbed denticles leave no pediments but pro- duce large spaces in the body of the soute. Such spaces (FIG. 7, do) are seen when the acute is treated w1 th KOH and mounted either in glycerine or dehydrated and mounted in canada balsam. Absoll)tion of the denticle together with its pediment takes place on the ventral as well as on the lateral scutes and on the fin spines of the animal. In between the fully developed denticles fresh dentiole buds indicate their renewal. On the body skin, as no sufficiently young specimens were available, it has not been possible to ascertain whether the denticles appear first or the scutes. But on the ventral skin denticle buds make their appearance earlier than the scutes. As for instance in Plecostomus plecostomus L., 65 mm. long, denticle buds are seen in various stages of development. But 42 before the denticle erupts, the scute has appeared and supports it. No stages were available to show that the scute is de- veloped as an extension of the denticle base or its pediment. The first step in the formation of the denticle is a crowding of cells of the external cerium beneath the basement membrane as the dentine germ, over which the basal, or palisade layer, of epidermis forms a oap of cells which become larger than the rest of the palisade cells (FIG. 23, ab) . The accu- mulation of mesoblastic cells produces a slight elevation against the base of the epidermis known as the dentine papilla. The epidermal covering forms the enamel organ which consists of: (a) columnar cells of the basal layer of the epidermis forming ameloblasts (FIG. 24, ~) and b), an outer layer of epidermal cells which have become flattened to form the ex- ternal epithelium of the enamel organ,a1 ea&HJ!ia UQklClz... (FIG. 24, ~). The mesoblastic cells lying in the axis of the papilla do not at .first show any regular arrangement, but soon they arrange themselves to form odontoblasts (FIG. 24, ob), which lie over a wide pulp cavity containing connective tissue fibres and connective tissue cells (FIG. 24, pea). The enamel organ grows over and embraces the denticle up to its base. It is interesting to note that the enamel organ is of uniform thick- ness throughout its length, (FIGS. 24 and 25, ~). 43 The dentine is the first to be formed between the layers of ameloblasts and odontoblasts, and by its formation the dentine papilla grows in length while its apex grows by the formation of the enamel. In developing denticle denser part of the dentine owing to the presence of lime salts is clearly differentiated from the next layer of the "tissue on border land of calcification" (TOMES 1923), which is situated between the fully developed dentine and odontoblasts. This layer of semi-transparent substance (FIG. 26, ~) is known as "odonto- genic zone", according to MUMMERY (1924) or the "dentogenetic zone" of TOMES ( 1923). This zone takes a pale pink stain while fully calcified dentine turns a deep purple colour with Delafield haematoxylin counterstained by eosin. As the denticle grows the pulp oavity1 extends into the enamel (FIG. 26, pob). The base of the denticle below the point of extension of the enamel organ elongates, as soon as the denticle is fully developed, into a transluscent calciferous connective tissue towards the underlying plate of bone and joins to it (FIGS 26 and 30, .J2.!!). The tip of the denticle (1) In developing denticle the prolongation of pulp cavity into the enamel, continuation of Nas.myth's membrane with the basement membrane and transluscent tip of dentine cone (FIG. 25, d) where enamel is being formed seem to lend support to KERR's views (1903, 1924) that enamel is formed by conversion of outer layers of dentine. 44 then pierces the epidermis. It is interesting to note that though the ameloblasts have disappeared the external epithel- ium of the enamel organ persists and is carried with the tip of the denticle and remains attached to the epidermis when the denticle has pierced it, (FIG. 30, ~). The enamel epithelium is, therefore, a distinct structure from Nas.myth's membrane which Olosely invests the enamel, and is continuous with the basement membrane, while the enamel epithelium is seen extending down to the junction of the denticle to its pediment, and lies externally to the layer of ameloblasts. Formation of the pediment has not been observed by any of the previous investigators of the dentioles of Lorioariidae except by CARTER (1919) who in the case of Pseudacanthicus serratus remarks that the condition is identical with that seen in certa~n of Gadidae "where the tooth, the pediment and the connecting substance develop on the surface of one dentinal papilla.'' Present investigations sbJw that in Plecost6mus plecostomus, L., as soon as the denticle has pierced the epidermis the pediment makes its appearance as a calcification in the middle of the transluscent calciferous prolongation of the denticle base:; as a ring-shaped structure at the base of the denticle and extends :f:.'towards (FIGS. 27 and 28, ,ES) and joins the underlying plate of bone. The pediment does not join the body of the denticle, and the portion of the transluscent zone which remains between the two does not 45 calcify but persists in the adult as connective tissue fibres on both sides of the pediment for the attachment of the den- ticle (FIG. 27, .£!!). The significance of the pediment lying between the connective tissue fibres will, thus, be apparent, as it develops in the connective tissue matrix which is in direct prolongation of the dentinal papilla, as a distinct calcification. The denticle, pediment and the connecting zone are therefore, formed on the surface of the same papilla. At first the line of termination of the base of the denticle is straight (FIG. 30, b) but gradually the inner surface of the connecting zone becomes converted into dentine and fonns short prolongation into the cavity of the pediment. CARTER (1919) describing the development of a tooth of a Sea Bream and a Blenny remarks that he has followed out a complete cytomorphosis of the cells on the surface of the denti- nal papilla in several Teleostean Fishes and finds "that the changes through which they pass are identical whether they go to form_ the denticle, the pediments or the connecting area". He sums up briefly that "the tooth with its pediment develops deep in the tissues as a single entity and not until it has assumed its functional position does its pediment enter into connection with those of the adjacent teeth to farm a continu- ous supporting bone, the tooth invariably retains its charac- teristic dentinal structure, but the pediment in its growth 46 undergoes a gradual progressive transition both on its inner and outer surface from dentine to bone and becomes incorporated in the substance of the bone". He further adds that "since tooth or denticle, pediment and connecting area are formed on the surface of the same papilla it seems certain that the difference in structure seen in the fully developed hard. tissue is due to same influence exercised by the investing tissues. Over the area invested by epithelium the typical dentinal structure obtains, where the investment is mesodermal there is an approximation to bone, whilst at the point of junction of epithelium with mesoderm a layer is formed which when calci- fied presents a glassy appearance." To this explanation given by CARTER (1919) as to the difference seen in the structure of the denticle and its pediment it may further be added that tbe body of the denticle is invested externally by ameloblasts and internally by odontoblasts and there is no possibility of the inclusion of any scleroblasts as bone cells, while the pediment, which is not bounded either externally or internally by any definite layer of cells and is surrounded by connective tissue cells, during its calcification and growth towards the under- lying plate of bone encloses some of the surrounding sclero- blasts as bone cells and becomes bony in its structure. The denticles appear one after the o~her antero-posteriorly, those on the carinate portion of the acute develop earlier than the others and their pediments join with one another at their bases by bony trabeculae to form a continuous covering over the posterior surface of the scute. At their distal ends the pediments are free from each other and form cup-like elevations on the surface of the underlying plate of bone (FIG. 30, .I?•) Fin skeleton The pectoral spine shows several rows of movably articulated denticles which are inclined towards its free distal end, but those near the free tip are large and are curved so as to face the base of the spine. The denticles are largest on the ex- ternal border and decrease in size on the dorsal and ventral surface of the spine. On the pectoral spine of P. commersonii but not in P. plecostomus, there is on the internal border one single row of stout denticles just at the junction of the fin membrane (FIG. 33, i). There are no serrations as are found in other members of Siluroidea. The spine of pectoral and dorsal fins consists of hard bony structure enclosing a central cavity (FIGs. 34 & 35, .£.£!) with a lamellated isopedine layer (is), a middle layer of Haversian canals (.!!£) and an outer layer formed by the fused pediments of the denticles (FIGS, 34 & 35, d). All the other fin rays are segmented, divide into fine branches towards the periphery and are also covered with denticles. The first rays of other fins consist of hard inflexible bony substance at the base, but become flexible towards their free extremity, 48 In a longitudinal section of the first ray of pelvic fin towards its basal part (FIG. 3? ) can be seen plates of bone (.te,), with six to ten lamellae of isopedine, Haversian canals and numerous bony corpuscles surrounding a central canal which is filled with connective tissue, fat and blood vessel. The plates lie close together immediately beneath the outer layer of the corium, and are joined by connective tissue fibres (!.£) which run from the margin of one plate to another. They carry on their surface denticles which are socketed as on the scutes. The pediments project into, and are covered by, the outer layer of the cori um. Towards the periphery (FIG. 36 ) the plates (.f.J2.E.) are thin with hardly more than one or two lamellae of isopedine and the blood vessels enter directly into the cavity of the pedi- ment. The plates at the periphery, therefore, resemble in their structure the scutes on the ventral Surface of the body. Their structure leaves no doubt that the periphery of the fin is the seat of formation of fresh plates as has also been observed by HERTWIG (18?6) in HyPostoma.. The fin spines and the fin rays at the periphery are prolonged into bundles of fibres of perfectly homogenous structure, and the germ of the fin platelets begins with the calcificatio.n of the inner layer of the corium.. The denticle with its pediment develops (FIG. 36 & 37, ~) independently and joins to the underlying plate of bone in the same way as in the case of the body scutes. 49 By further calcification of the deeper layers of the cerium the plates are. thickened while the conneoti ve tissue in between them remains non~calcified and connects the plates together (FIG. 36 & 3?, fc). Ancistrus KNER. (Syn. Hypostomus c. & V.). Ancistrus is anothe:r genus belonging to the sub-family Plecostominae and differs from Plecostomus in the structure of the operculum and interoperculum, which are independently movable. The lower surface of the head and abdomen in the adult is either completely covered with small granular scales or is naked (REGAN, 1903). Ancistrus multiradiatus RANCK. Integument. There are neither club cells nor sac cells in the epidermis at any stage. In 21 mm. long specimen (FIG. 43 ) the epidennis consists of (a), three to four layers of epithelial cells, the basal layer (epb) having oval cells with large nuclei, while those of the upper layer are slightly flattened; (b), mucus cells (mo) which are pear-shaped with nuclei lying at the base and opening to the exterior by a short neck; (c) sensory cells which are of two kinds (1) nerve hillocks (FIGS. 38 & 39) similar to those seen in Plecostomus, consist of two or more cells, large in size with large round nuclei and with acidophil contents (ss) staining pinkish with eosin and acid fuchsin. They lie in a oavi ty and have a base 50 of epithelial. cells {,!£); (ii) the end buds or the pear-shaped sensory organs as found in Plecostomus; (d) fibroblasts or connective tissue cells are present in the lower layers (FIG. 43, fi). The epidermis is separated from the cerium by a basement membrane. The corium on the ventral skin in 10 and 21 mm. specimens does not show any sharp differentiation into two layers but it does so on the body where the scutes are develop- ing: the outer layer is formed of loose network of fibres and the inner of fibres running parallel to the inner border of the scute in a horizontal plane (FIG. 40, .£2 & ci). In 21 mm. specimen the ventral skin is without any ossi- fication while on the body scutes have appeared but do not fo:rm a complete armour and are not fully covered with denticles. The formation of the denticles as in Plecostomus commences from the carinate portion in an antero-posterior direction and spreads to the sides of the acute. .Anteriorly there are three rows of denticles one on each series of the scute, while posteriorly, two more rows are appearing. On the remaining part of the scute the denticles have not yet appeared. Those on the carinate portion of the scute (FIG. 44, d) are three to five in number and are directed backwards, and increase in size antero-posteriorly. In all cases the posterior denticle which is the largest is still developing (FIG. 44, dd). The 51 structure of the denticle is the same as in Plecostomus and they develop in the same manner. The scute makes its appearance independently of denticles and appears as a thin strip of calcified tissue in the inner layer of the corium and is surrounded by scleroblasts. The lamellae of the scute merge into the connective tissue fibres of the corium on the borders (FIG. 41, bsp}. The dent icle makes its appearance in the outer layer of the corium. Its base elongates and forms the connecting area and the pediment which joins by bony trabeculae with the neighbouring pediments and form the upper layer of the scute (FIG. 40, _E.). The growth of the acute continues by development of fresh denticles at the posterior border of the scute and calcifica- tion of the deeper layers of the corium. The denticle, the pediment and the connecting area develop as in Plecostomus on the same dentinal papilla and the difference in structure of the pediment from the body of the denticle is illustrated in FIG. 42 -which shows pediments of two denticles, with sclero- blasts (.£1£) lying at their borders and same have become en- closed as bone cells in the substance of the pediment itself {be). - Lipopterichthys NORM. Lipopterichthys (NORMAN, 1935) is a recently described genus belonging to the sub-family Plecostominae and is closely structure of the denticle is the same as in Plecostomus and they develop in the same manner. The scute makes its appearance independently of denticles and appears as a thin strip of calcified tissue in the inner layer of the corium and is surrounded by scleroblasts. The lamellae of the scute merge into the connective tissue fibres of the corium on the borders (FIG. 41, bsp). The denticle makes its appearance in the outer layer of the corium.. Its base elongates and forms the connecting area and the pediment which joins by bony trabeculae with the neighbouring pediments and form the upper layer of the scute (FIG. 40, .P,). The growth of the acute continues by development of fresh denticles at the posterior border of the scute and calcifica- tion of the deeper layers of the corium. The denticle, the pediment and the connecting area develop as in Plecostomus on the same dentinal papilla and the difference in structure of the pediment from the body of the denticle is illustrated in FIG. 42 -which shows pediments of two denticles, with sclero- blasts (ctc) lying at their borders and some have become en- closed as bone cells in the substance of the pediment itself (be). Lipopterichthys NORM. Lipopterichthys (NORMAN, 1935) is a recently described genus belonging to the sub-family Plecostominae and is closely 52 related to Chaetostomus TSCHUD., but is without an anal fin. Lipopterichthys carrioni NORM., is the only representative of the genus. In a 60 mm. specimen (FIG. 45, ~) the epidermis cainsists of : (a} seven to eight layers of epithelial cells, the basal layer forming the palisade layer of columnar cells, and the superficial layer contains flattened cells, while the cells in the middle layer are oblong or polygonal; {b) the gland cells consist of mucus cells only. There are no club cells and no sac cells. The mucus cells (FIG. 46, .!!1£) are not goblet-shaped but are bag-like or pear-shaped and their contents form a loose network with the nucleus lying at the basal wall. They stain violet with methylene blue and tol~idin blue; (c) the sensory cells are of the usual pear- shaped form., but on the ventral sld.n are very scarce; ( d) fibroblasts are oonfined to the lower layers (FIG. 45, fh). The corium as usual consists of two layers: the outer and the inner, with a few scleroblasts and a few blood vessels. The abdomen and the ventral surface of the head are with- out any ossifications. The body is covered with five longi- tudinal rows of plates at the caudal peduncle and four anterior- ly. They have the same shape and form as in Plecostomus and are covered over with rows of movable denticles on their pos- terior part, while anterior part is overlapped by the posterior part of the preceding scute. The tips of the denticles on the lateral scutes are pointed, while those on the ventro-lateral parts are dorso-laterally flattened. 53 The spines and the rays of the fins are also covered over with denticles. On the ventral surface of the pectoral and pelvic spines and of the ventral ra:y of caudal fin the enamel tips are dorso-ventrally flattened (FIG. 48, ~), while the denticles on the upper surface of the spine and the fin rays have normal pointed tips (FIG. 47, ~). The structure of the denticle is the same as in Plecostomus. The enamel covering in a pointed denticle ex- tends to one-third of the total length of the denticle while in a flattened tip it extends to one-half mhe length. The . body of the denticle consists of hard tubular dentine, the tubes extending to the base of the denticle. The pulp cavity (FIG. 47, pea) stops short below the am.elo-dentinal junction and gives off branches which penetrate into the enamel to one-fourth of its length (FIGS. 47 & 48, pch). The base of the denticle is slightly socketed. In a developing denticle from the ventral surface of the pectoral fin spine, the flattened ena.tml tip is fully developed, while the ped_iment is still lacking, and the base of the denticle is prolonged into coIU1ective tissue fibres (FIG. 49, .PJ!). Xenocara RGN. Xenocara, another genus belonging to the sub-family Plecostominae, is distinguished from Ancistrus by having the snout with a naked margin. The abdomen is naked, the head has no ridges or protruberances and is flat above. The scutes 54 are not carinate. In 90 mm. specimen of Xenocara hoplogenys GTHR., the structure of the integument is very similar to that of Lipopterichthys and the epidermis consists of numerous bag like mucus cells but no granulated cells (FIG. 50, ~). The mucus cells have network like contents which are readily stained with methyl blue and toluidin blue. Some of them are empty. Fibroblasts are found in abundance in the lower layers of the epidermis (FIG. 50, f h ). The scutes and the denticles have the same structure as in Plecostomus. The denticles are attached to their pediments by fibrous tissue and are socketed. Those on the ventral sur- face of the pectoral and the pelvic spines and on the ventral surface of the scutes on the caudal peduncle are flattened dorso-ventrally, but their tips (FIG. 52, ~) are pointed and not round as in Lipopterichthys. The base of the denticle on the pectoral spine is in contact with the rim of the pediment (FIG. 51, l?,) and their movement is thus very much restricted. Th~ bristles (FIG. 53) on the operculum are elongated denticles which are firmly anchylosed to their pediments without any perceptible movement. The ground section of the bristle (FIG. 53) shows the enamel tip (!_), the tubular dentine (de), the tubes extending to the base and the p~lp cavity, which is prolonged into the enamel, gives three or more branches (pob). 55 The pediment (~) is bony with numerous bone corpuscles. The base of the bristles is in contact with the elevated rim of the pediment and the connecting zone is calcified (FIG. 54, ~) into a transluscent hard substance in which faint outlines of its fibrous structure are still visible • . This transluscent tissue is richer in organic matter than the dentine and bone and takes stain more deeply than either of the two. In ground section with picro-indigo carmine the dentine stains light blue, the bone greenish-blue and the connecting zone deep blue. Eosin stains it deep pink while bone and dentine are only slightly stained. The pediments bearing the bristles are embedded in plates of bones which are united with each other by connective tissue fibres and are in continuation of series of plates on the ventro-lateral border of the operculum, anterior to the oper- cular aperture. The plates anterior to the bristles bear ordinary movable denticles. The bristles are, therefore, denticles which have become firmly fixed to the pediments by the calcification of the connecting area. Pseudacanthicus BLKR. Psaudacanthicus is another genus belonging to the sub- family Plecostomin~e. In a 300 mm. long specimen of Pseuda- canthicus serratus c. & v., the body is covered by five longi- tudinal rows of scutes. Ventrally the head and abdomen are 56 naked except at the bases of the pectoral fin where there are l? to 20 granular scales with one to five firmly fixed denti- cles on each. The scute as in Plecostomus consists (FIG. 55, bs) of a basal layer of lamellated isopedine (is) containing bone cor- puscles with ramifying canaliculi; a middle vascular layer of Haversian canals (.!!£) and an upper layer formed by the coalascence of the pediments(~) of the denticles, and is thick- est at the anterior (FIG. 35, bsa) and tapers towards the posterior border (..!2!E,). On the lateral body scutes the number of rows of denticles is very much reduced varying from 4 to 6 consisting of one median row of 3 to 4 firmly fixed denticles and 3 to 5 rows of small denticles most of which are freely movable in their sockets (FIG. 59). The denticles at the posterior border of the scute though elongated are still movable while at the anterior border they are firmly fixed (FIG. 56). The denticles of the middle row of the scute are very much elongated and vary from 4 to 6 mm. in length and 1 mm. in thickness at the base and are slightly bent backwards (FIG. 59, d). The ventro-lateral parts of the body scutes, extending between the bases of the pectoral and the pelvic fins and em- bedded in the skin bear no denticles. Seven scutes behind the first dorsal fin up to the base of the second dorsal fin have no denticles, nor are there any on the scutes behind the 57 base of the pelvic and anal fins at their ventro-lateral parts. Thus all those parts which are likely to be covered by the fins when they are folded are devoid of any denticles. The denticles on the dorsal, the pelvic spines and their ra:ys and the anal fins are not enlarged. It is only on the outer dorso-lateral border of the pectoral spine (FIG. 62, d) that the denticles are elongated and firmly fixed, and increase in length from the base to the tip of the spine from 5 mm. to 9 mm.. On its ventral surface they are small, measuring 2 to 4 mm. in length. In between the firmly fixed denticles there are movably articulated ones. Those at the extreme tip of the spine are freely movable. They are sharp pointed, erect with their tips curved so as to face the base of the spine. The denticles as in Plecostomus consist of hard tubular dentine with an enamel covering at the tip. In a l•l mm. long denticle the enamel extends over 0•5 mm. of the surface of the dentine, while the tip of the enamel measures O • 24 .mm. Under the action of the acid the concentric rings appear extending to half- the length of the denticles. Nasmyth' s membrane per- sists after the action of the acid but falls off if the denti- cle is stirred from its place on the slide (FIG. 58, nm). The enamel is distinctly tubular (FIG. 57, ~) and forms a cap over the cone like tip of the dentine. The pulp cavity narrows at the tip of the denticle and gives two branches or more into the enamel (FIG. 58, pch). 58 The dentine tubes extend up to the base of the denticle, which, as in Plecostomus, is prolonged into short extension and is socketed, and is attached to its pediment by connective tissue fibres, which in decalcified sections are distinctly seen ascending into the substance of the dentine on one hand and descending on the other into the substance of the pediment, (FIGS. 60 & 61, .QJ!). In firmly fixed denticles the connective tissue fibres, attaching the base of the denticle to the exter- nal wall of the pediment are still of the same structure as in Plecostomus, while the fibres between the basal extension and the internal wall of the pediment have become almost trans- luscent (FIG. 56, ..£2.!!) like those seen in the anchylosed br.tstles of Xenocara. The connecting zone has calcified though there are neither bone cells nor any dentine tubes, and is quite hard and immovable. There are faint striations in its structure which show its original fibrillar character. CARTER (1919) describes the firmly fixed or anchyloseddenticles on the post-temporal bone of Pseudacanthicus serratus, and remarks that their "attachment to the pediment is effected by a layer of almost structureless calcified tissue which occu- pies the area of junction". Some of the firmly fixed denti- oles on the scute and pectoral spine are not yet completely anchylosed as slight movement is perceptible if examined under the binocular. 59 The pediment:, of the neighbouring denticles are connected together by bony trabeculae, and are raised above the level of the scute with their anterior border higher than the posterior so that the denticle is directed backwards (FIG. 55, d). At the posterior border of the scute the thickness of the scute is represented by the pediment with 2 to 3 lamellae of 1so- pedine. Where the denticle is lost by absorption a gap is produced on the swrface of the acute (FIG. 55, .£2)• The course of the bony lamellae of the pediment is quite different from those of the scute (FIG. 56, ~), as they run vertically to those of the scute and are in the same plane as those of the connecting zone. This fact is further evidence for the entity of the denticle and its pediment. The operculum is studded with rows of bristles which vary in length from 3 to 15. mm. They are not anchylosed to their pediments (FIG, 63, br) but are socketed and are borne on small plates of bones each having 2 or more. The plates, like those on the ventral skin of Plecostomus, are joined with connective tissue fibres with one another. The bristle (FIG. 64} consists of en enamel tip, hard tubed dentine body and a pulp cavity, which is broad at the base and is prolonged into the enamel to half the length of the tip. I • 60 Sub-Family Hypoptomatinae. Otocinclus COPE The genus Otocinclus belongs to the sub-family Hypopto- matinae. The body is enclosed in bony scutes which form five longitudinal series anteriorly on each side of the caudal peduncle and three posteriorly; the lower transverse parts of clavicle and coracoids are exposed and appear as two pairs of plates between the bases of the pectoral. The abdomen is covered either by three longitudinal series of plates or with numerous plates irregularly arranged. Integument. No account of the structure of the integu- ment of Otocinclus is available excepting a brief foot-note by RAUTHER (190? ,. p. 522) that in Otocinclus (O. notatus and o. vestitus), "I miss the pear-shaped Kornerdrusen, only slime glands are present, the deeper layer of epidermis is permeated by rich system of lymph vessels {System von Lymphgefassen)". The study of the structure of the integument of Otocinclus nigricauda BLGR., 21 mm. in length and of Otocinclus vittatus RGN., 19 mm. in length do not corroborate the statement of RAUTHER (1919), as in both these specimens the epidermis is rich in sac-cells or granular cells {Kornerdrusen) and various stages in their development can be studied. In o. nigricauda (FIG. 65), epidermis consists of : (I) six layers of epithelial cells, basal layer consisting of columnar cells (FIG. 66, ~, 61 upper layer of flattened cells and in the middle the cells are either oval or polygonal (FIG. 66, epc). The nucleus in all the cells is large with distinct nucleolus; (II) the mucus cells, which are situated in the upper layers of epi- dermis and open to the exterior. They have a typical goblet shape with nucleus lying near the basal wall and are stained with mucin stains; (FIGS. 66, 67, !!!£); (III) the sac-cells or granulated cells (Kornerdrusen), which are bag-like lying in the middle of the epidermis with a long or short neck which opens to the exterior (FIGS. 66, 67, .s.2}. The nucleus (!!£) lies towards the base of the cell. Their contents are dis- tinctly granular and do not stain with Delafield haematoxylin but are acidophil and stain with eosin and acid fuchsin. In their early stages t .he sac ... cells occupy almost the whole of the thickness of the epidermis and rest directly on the corium. Their contents are almost wholly homogenous and are deeply stained with eosin and acid fuchsin (FIG. 68, ,6!}• The nucleus lies towards the base of the cell, is large, round and has a large nucleolus. Rarely there are two nuclei. In some cells (FIG. 66, .B.£) the contents have contracted and their sides are empty. As soon as the cells open to the exterior, the plasma is transformed into granular substance, the nucleus shrinks towards the basal wall and the whole cell seems to decrease in size. The various stages seen in the ventral and body skin leave no doubt that these large cells with the deeply stained 62 acidophil plasma have nothing in common with the club cells found in other fishes. Their staining behaviour, their basal nucleus their structure with long neck opening to the exterior, and finally the intermediate stages which show that after their opening they lose their dense plasmatic structure of their con-. tents and become granular are characters such as to distinguish them from the club cells and entitle them to be included in the category of sac- cells Kornerdrusen" of RAUTHER (1907) and "Sackformige Serose Drusen" of STUDNICKA ( 1906, a) J • On the body of o. nigricauda the mucus cells are in abundance while the sac cells are fewer than their number in the ventral skin; (IV) fibroblasts, which are found in abundance in the lower layers of the epidermis (FIGS. 66, !,2); (V) the pyriform sensory organs or end-buds, which are similar to those found in the rest of the family. The structure of the epidermis is the same in 19 mm. specimen of o. vittatus. The sac cells are found in different stages of their development (FIG. 70, .6.£). The mucus cells are present in the upper layers and fibroblasts (FIG. 70, fb) are much more abundant in the lower layers of epidermis than in o. nigrioauda. The epidermis is separated from the corium by a basement membrane. The corium. consists of an outer and an inner layer. The outer layer as usual is loose in its texture, contains blood vessels, pigment cells and a few connective tissue cells. 63 The inner layer (FIG. 65, ci) is fibrillated and contains blood vessels and connective tissue cells. The acute forms boundary between the two layers, (FIG. 65, bsp}. The structure of the scute (FIG. ?l ) is the same as has been described for Plecostomus, and of the denticle (FIG. 73 ) is also the same as in Plecostomus, excepting that the enamel is not deep brown but is white and extends over half the length of the denticle and forms a pointed conical cap over the den tine, which does not form an arch as in Plecostomus at amelo-dentinal junction, but is prolonged as a triangular wedge into the enamel cap (FIG. ?4, ~). The pulp cavity is prolonged into the wedge of the dentine and gives 2 or 3 branches into the substance of the enamel (FIG. 73, 74, pcb). The dentine is tubular and the tubes extend right down to the base of the denticle. The denticle is attached to its pediment in the same manner as in Ple cost omus, but its bas al prolongat ion i s not long an~ i t is socke t ed very slightly. Sub-Family Loricar iinae. Loricaria L. The upper part of the body is protected by bony scutes, the caudal peduncle with paired dorsal and ventral series of scutes , and posteriorly , at least , with only a single lateral 64 series ·of scutes; the lower part of the clavicle and coracoid is not exposed. In Loricaria labialis BLGR., the lower surface of the head is naked, except for a patch of small granular scales in front of each gill opening. The abdomen has one or two series of plates between the lateral series and the anal plate is bordered anteriorly by 2 or 3 plates. Loricaria cataphracta L., has the lower surface of its head naked, the abdomen (FIG. ?6, ~) has 3 to 5 series of plates between the lateral series, anterior plates are small and numerous, and there is no distinct enlarged anal plate. In Lorioaria typus BLKR., the lower surface of the head is naked, the abdomen has two series of plates between the lateral series and the anal plate is bordered anteriorly by 2 or 3 plates, (REG.AN, 1903). Integument. In L. cataphracta there are neither club cells nor sac cells. The epidermis (FIG. 75, !:£) consists of (1) six to eight layers of epidermal cells, the basal layer having columnar cells; (2) the mucus cells (me) which open to the exterior by short necks. RAUTHER (190?) in L. lanceolata, measuring 10 cm., records the absence of club cells and mentions the presence of numerous beaker-shaped cells; (3) the cutaneous sensory cells, which in an adult specimen of L. cataphracta (FIG. 75, nb) are present in the form of end-buds as seen in Plecostomus. In L. lanceolata, 10 cm. in length, RAUTHER (1907) descrtbes groups of 2 or 3 cells, large in size, oval in shape, 65 with granulated acidophil contents and large nucleus, like the sensory hillocks described in Plecostomus and Ancistrus. He recognises them as "free accessory lateral sense organs"; (4) fibroblasts, (fb) which are abundant in the lower layers. The cerium is separated from the epidermis by a basement membrane and consists of two layers as usual. Dermal Skeleton In a 50 mm. specimen of L. cataphracta ventral scutes have not developed, in 73 mm. long one, a few scales have appeared and lie scattered, and in 240 mm. long they are rully developed. The scutes on the abdomen do not overlap each other but are joined by connective tissue fibres and lie at the boundary between the outer and the inner layer of the cor.ium (FIG. 76, ~). They are studded with movable denticles. The simplest scute (FIG. 78, vs) on the ventral skin, consists of a single denticle, its pediment and a thin plate of bone which lies at the base of the pediment. The denticle (FIGs. 78 and 79, d) is freshly developed as its base is not yet socketed,. At one side of the scute is an opening for the blood vessels which enter the cavity of the pediment and supply the pulp cavity (FIG. 78). The single denticle scute is similar to the early stage of the ventral scute of Pleoosto- 66 The body scute, as ih Plecostomus, consists of a basal layer of lamellated isopedine (FIG. 80, ~}, the laimllae running in a horizontal plane; a vascular layer (he) end an upper layer of pediments on the uncovered part of the scute, while the covered anterior part has a lamellated bony structure. WILLIAMSON (1851) described the structure of the scute of L. cataphracta L., as composed of a "truly osseous texture closely resembling that seen in Acipenser and Lepidosteus, and consisting of numerous lamellae, arranged like those of Lepidosteus". The features in the structure of the scute of L. cataphracta resembling those of Acipenser and Lepidosteus as enumerated by WILLIAMSON (1851) are : (1) truly osseous texture composed of lamellae arranged in a similar way and with many large Haversian canals; (ii) the lamellae at the posterior margin of the scute run parallel to the inferior surface but at its anterior portion where it has supported the free edge of its anterior neighbour, the lamellae turn upwards and are ·prolonged over a portion of the upper surface; (iii) numerous quadrate lacunae with their divergent canali- culi spread out in layer between the lamellae; and (iv) an abundance of lepidine tubes penetrating the acute from below. The structure of the corium, in Loricariidae, as has already been stated (page 2S' ) is similar to that described - ' 67 by NICKERSON (1893) in Lepidosteus, and as the scutes develop in the corium, the resemblance between the structures of the scutes of L. cataphracta and Lepidosteus can be explained as due to similar process of calcification: viz. (i) as the scutes are formed by the calcification of the fibres of the inner layer of the corium, which run parallel to the surface of the epidermis in a horizontal plane, the course of the lamellae in both cases is similar; (ii) during the calcifica- tion the lamellae of isopedine enclose within themselves the scleroblasts, which become the bone corpuscles and become dis- tributed in the substance of the scute and have the same structure in both cases with a system of ramirying oanaliculi; which spread out in the direction of the lamellae of isopedine; (iii) the lamellae of isopedine merge into the substance of corium at the borders and those at the anterior border of the scute on account of the arrangement of scleroblasts and the deposition of the calcified substance appear to turn upwards and spr~ad on the upper surface of the acute; (iv) lepidine tubes of WILLI.AMSON (1849) are nothing else but the non- calcified connective tissue fibres which are visible in ground sections. In Lorioariidae, however, there are neither lepidosteoid tubes nor any ganoin, the scutes, therefore, differ histologi- cally from that of Lepidosteus and Polyprrus. Similarities in the dermal skeleton of Loricariidae, Ganoids and others, 68 as explained by GOODRICH (1909) and LEEGE (1922} are due to convergence. The denticles (FIG. 80, d) have the same structure as in Plecostomus, with an enamel tip and tubular dentine, the tubes extending to the base of the denticle (_g!). The pulp cavity gives two or three branches to the enamel. The base of the denticle is socketed and its attachment to the pediment is effected by connective tissue fibres as in Plecostomus and not by "a capsular expansion of the membrane which covers the sur- face of the scale" as believed by WILLIAMSON (1851). Sub-Family Argiinae. Arges c. & V. REGAN (1903) regards the sub-family Argiinae as "degraded or ultra specialized forms closely related to the Neoplecostomus, frcm which they differ mainly in the naked body and the stronger ribs". Integument. No account of the structure of the integument of Arges- is available. If the theory propounded by RAUTHER (1907) as to the frequency of club cells in relation to the skin skeleton were true, one would expect from the nakedness of the body of Arges to find the structure of the integument similar to that of the naked Siluroidea. But the case is not so. The epidermis totally lacks the club cells which are so character- istic and regular a feature in the skin of naked Siluroidea. 69 In Arges brachycephalus GTHR., measuring 50 mm. in length the epidermis consists of : (FIGS. 81, 82, .!ll?,) (1) six to nine layers of epithelial cells, the basal layer forming a palisade layer of elongated cells. The total thick- • ness of the epidermis measures 0.095 mm., while tbe length of the cells or the palisade layer measures 0.036 mm. The pali- sade · layer, thus, .occupies almost one-third of the thiclmess of the epidermis, and performs the ftm.ction of support, a func- tion which in naked Siluroidea is performed by large club cells. The palisade cells are ri eh in plasma, with a large nucleus lying almost in the .middle of the cell. The apex of the cell . is wedged in between the epidermal cells of the upper layer. The cells of the remaining layers of epidermis are either oval or polygonal with their nuclei lying not in the middle but slightly towards the base; (2) the mucus cells with long necks, (FIG. 83, .!!!.£), which are large and sac-like, and in most of them the contents show a network structure which readily take a violet stain with methylene blue and toluidin blue. In others the network is faint, and some of them are empty. They lie in the upper layers of epidermis, but the longest ones, which vary in length from o.035 to 0.043 mm., that is to say almost equal to or even longer than the length of the palisade cells, lie immediately over tbe palisade layer. Those lying in the deeper layers are also provided with long necks which open to the exterior; 70 (3) the fibroblasts, which are present in the lower layers (FIG. 82, fb); (4) the pigment cells or chromatophores of REID (1894), which send their .ramified processes among the inter-epithelial spaces and are found at various levels (FIG. 84, R,£); (5) the cutaneous sensory organs, Vllli.ich are of two types as found in the other members of the family, the end buds (FIG. 86, BJ?.) and the nerve hillocks (FIG. 85). The corium (FIG. 81, .£) is separated from the epidermis by a basement membrane and consists of not very sharply defined layers, an outer and an inner. The inner layer is composed of horizontal fibres with a few vertical bundles. There are a few blood vessels and a few connective tissue cells. The pig- ment cells (.E.£) lie just beneath the basement membrane on the body. Below the corium is the adipose .tissue (FIG. 81, at) which is followed by anot her layer of pigmen t cells , (FI G. 81, 1?..£ ) • Dermal Skeleton Arges does not possess any armour of bony plates on the body or on the ventral skin . They have, however, so me remnants of the character of Loricariidae. GUNTIIBR ( 187 0 ) , e nu.me rating the oharacters common to Arges and Loricariidae , points out that "the spiny covering of the first ray of the fins of Arges and Stygogenes are identical with the spiny armature of 71 Hypostomus and of some Loricaria", and considers "Arges as a naked Loricaria or vice versa Loricaria as mailed Arges". In Arges guentheri BLGR., the outer rays of the pectoral and the pelvic fins are thick but flexible and are covered over with small denticles. The outer ray of the pelvic fin is flat and has 9 to 10 rows of denticles on its lower surface, those situated ventrally are double the size of those on the border. On the pectoral fin the denticles are present on the outer border of the outer ray and are lacking on its upper surface and on the remaining ra:ys. There are denticles on the base of the ventral and dorsal rays of the caudal fin but none on the dorsal fin which has a weak outer ray. The denticles are directed towards the -posterior part of the body. In Arges brachycephalus GTHR., the outer rays of the dor- sal, pectoral, pelvic and anal fins are thickened ~t flexible and bear denticles. In a 60 mm. specimen the outer dorsal ray is 12 mm. long and the base up to 4 mm., has no denticles, but on the outer border of the remaining length it has four rows which converge to two rows at the free extremity of the ray. The region of fresh fonnation of the denticles is at its free extremity. The outer ray of the pectoral fin on its oute-r border and the outer ray of the pelvic fin on its ventral surface have 5 to 6 rows of denticles which converge to two rows at the extrem.tty. There are no denticles on any other ray of the fins, The upper and lower rays of the caudal fin also each bears denticles in four rows. 72 Structure of the denticle . The denticle (FIGS. 92 and 93), as in Plecostomus, con- sists of dentine body with enamel covering, enclosing a pulp cavity and movably attached to a pediment. The yellowish- brown enamel(~) spreads over half the length of the denticle and forms a thick cap at the tip ' and is tubular in its struc- ture. The dentine (de) is tubular at the top for two-thirds of its length, while for one-third of its length it has no trace of any dentine tubes, and instead shows a faint fibrillar structure in ground sections. The dentine tubes (FIG. 93, dt), in the upper part branch out from the pulp cavity and spread towards the border but stop short just below its surface, and are joined together by network of their ramifications. Gradu- ally the tubes dimin.ish in length and in number towards the base and ultimately disappear totally from the basal part of the den tic le (FIG. 93, b). The pulp cavity is wide at the base but stops abruptly below the dentine tip and gives off 6 to 10 branches which traverse the remaining length of the dentine and pass to a vert slight extent into the enamel (FIG. 93, pea and pcb). At the base the pulp cavity opens into the cavity of the pediment. The pediment (FIGS. 92, 93, 12.) has a ring-shaped bony structure, The base of the denticle (FIG. 93, b) is not pro- longed into a basal extension as in other members of the family Loricariidae and the denticle, therefore, is not socketed. Its / 73 attachment to the pediment is effected by connective tissue fibres (FIG. 93, .£!!). Fresh developing denticles on the pelvic fin (FIGS. 88 and 89) show a similar mode of development to that in Pleco- stomus. The enamel tip is fully developed, while the base of the dentine is prolonged into connective tissue fibres (FIG. 89, ,E.!)• The calcification of the fibres continues to form the non-tubular part of the dentine. The pediment develops below the base of the denticle leaving a zone of connective tissue fibres as the connecting area (FIG. 88, E and~). In a transverse section of the outer ray of the pectoral fin (FIG. 90) the arrangement of the denticles and their re- lation to the underlying-plate of bone is well seen. There is a central cavity (cov) which is bounded by 4 series of bony .- plates, two dorsal, forming the outer border of the ray and two ventral, forming the inner border. The epidermis (~) has the same structure as on the body of the animal, but the corium is sharply divided into two layers the outer (.£2) and the inner (ci); the outer consists of a network of loose fibres and the inner contains longitudinally running connective tissue fibres. The bony plate forms the boundary between the two layers. The plates (FIG. 90, .£.E.) on the outer border of the ray bear denticles and resemble in shape and structure the ventral skin plates of Pleoostomus, their basal part consists . . ?4 of lamellated isopedine (FIG. 90, is), the middle part con- tains the Haversian canals (he) and the upper part is com- posed of the pediments of the denticles (~). The denticles (d) pierce the epideimis which covers their basal part and extends up to the point of their junction with the pediment. The pediments are covered by the external layer of the corium. In a longitudinal section of the outer ray of the pectoral fin near its free extremity, (FIG. 91) the plates (~) are thin consisting of one or two lamellae of isopedine. The cavity of the pediments open into the central cavity (ccU) and thus receive blood vessels directly from the inner layer of the corium. The bony plates (fp') on the inner border are also thin. The plates are joined together by connective tissue fibres (FIG. 91, fc) which allow the free movement of the whole plate and thus contribute towards the flexibility of the ray. At the basal part of the ray (FIG. 92) both the outer and the inner plates (!l2) are thick, consisting of 4 to 12 lamellae of isopedine (is) with bone corpuscles with branching canaliculi. Each plate of the outer border bears 4 to 5 denticles. The same process of the development of the fin plate is seen here as has been observed in the case of Plecostomus. The increase in their thickness takes place by the calcification of the deeper layers of the corium. Argiinae though naked in their body possess an armature of true denticles on their outer fin rays. The denticles with minor ?5 differences have the same histological structure as in the other members of the family Loricariidae. The differences are, (1) the dentine is tubular in the upper part and hard and non-tubular in the basal part; (2) the denticle is not socketed, and (3) the pulp cavity itself is not prolonged into enamel but gives off branches which penetrate the enamel very slightly. REGAN (1903) has pointed out that the structure of the adipose fin in Argiinae which, in some species, is composed of a well-developed movable spine attached posteriorly by a mem- brane to the caudal peduncle, is suf'ficient evidence that the nakedness of the body is a "secondary character and not a primitive feature". Histological evidence may now be added to REG.AN's statement as the present investigations show that the structure of the skin of Argiinae is similar to that seen in the other members of the family Loricariidae and has no re- semblance to the structure of the skin as seen in the naked Siluroidea. Argiinae are, therefore, to be regarded as Loricariidae in which the nakedness of the body is secondary. They have lost their dermal skeleton but possess dermal denticles on their fin spines. Their skin still possesses the characteris- tic features of the family and totally lacks the club cells which in the naked Siluroidea occur as a regular feature and form 2 to 4 layers in the epidermis. , t 76 FAMILY CALLICHTHYID.AE The family Callichthyidae is generally defined as having "the body armoured with two rows of overlapping bony plates which, like the surface of the cranial bones, are beset at their hinder-edge with movable denticles." (GOODRICH, 1909). Examination of well-preserved specimens of many members of the family, however, shows that in addition to the denticles at the hinder-edge the upper surface of the scute is also covered over with a few movable denticles which are so loosely attached to their pediments that they ve-ry easily fall off if the animal is handled roughly. The scutes are larger anteriorly than they are posteriorly. In all the genera examined the lateral line, the existence of which according to LEEGE (1922) is denied by VALENCIENNES in Callichthyidae, commences at the base of the caudal fin and runs forward as a dermal canal until it reaches the second scute in Callichthys callichthys, the sixth - and not the fifth as mentioned by COLLIN~ (1895) - in Hoplosternum littorale and the third in Corydoras aeneus, which it enters and is conducted through the rest of the shields till it reaches the first and instead of entering the first scute it passes into a small canal bone. In the scutes which it pierces it has a small rol.llld opening at the anterior margin and a con- cave opening at the posterior border of the scute. C - l FAMILY C.ALLICHTHYID.AE The family Callichthyidae is generally defined as having "the body armoured with two rows of overlapping bony plates which, like the surface of the cranial bones, are beset at their hinder-edge with movable denticles." {GOODRICH, 1909). Examination of well-preserved specimens of many members of the family, however, shows that in addition to the denticles at the hinder-edge the upper surface of the scute is also covered over with a few movable denticles which are so loosely attached to their pediments that they ver-y easily fall off if the animal is handled roughly. The scutes are larger anteriorly than they are posteriorly. In all the genera examined the lateral line, the existence of wbich according to LEEGE (1922) is denied by VALENCIENNES in Callichthyidae, commences at the base of the caudal fin and runs forward as a dermal canal until it reaches the second scute in Callichthys callichthys, the sixth - and not the fifth as mentioned by COLLINGE (1895) - in Hoplosternum littorale and the third in Corydoras aeneus, which it enters and is conducted through the rest of the shields till it reaches the first and instead of entering the first scute it passes into a small canal bone. In the scutes which it pi~rces it has a small round opening at the anterior margin and a con- cave opening at the posterior border of the scute. ,L Callichthys L. Integument. No account of the structure of the integument of Calliehthys is available. HERTWIG (1876) investigated the dermal skeleton of Callichthys longifilis but the specimen had its epidermis torn off. RAUTHER (1907) has described the structure of Callichthys punctatus BLOCH., which is now recog- nised as Corydoras melanistius REG., and the structure of the integument in Callichthys differs from that of Corydoras. The two species possess specific characters in their epidermis which makes it easy to differentiate between the two.- In Callichthys callichthys L. 140 mm. long (FIG. 100) and in Callichthys pectoralis, ?4 mm. long, (FIG. 94), the epidermis consists of: (1) the ordinary epidermal cells which occur along the base of the epidermis and also in between the gland cells, the basal layer has conical or pyramid cells, while on the surface they are more or less flattened, and in the middle they are either oval, round or polygonal; (2) the mucus cells, which are present in the upper layers and open by wide pores to the exterior. They are not goblet- shaped but are bag-like or pear-shaped with broad base and narrow neck. Some of these cells are empty with the nucleus lying at the base, in others especially in the epidermis of Callichthys pectoralis, the cells have their contents in the form of 18se network (FIG. 94, me) and are stained d ep violet J. - 78 with Delafield haematoxylin, and violet by methyl blue and toluidin blue; (3) the club cells (Kolbenzellen) are large gland cells, not so far seen in Loricariidae, but forming a characteristic feature of almost all Physostomi. In Callichthys pectoralis (FIG. 94, .££) the club cells on the ventral skin lie in 2 to 3 layers and on the scute in one layer only. In the material of Callichthys callichthys the ventral skin is not well pre- served but on the body the club cells lie in single layer in the middle zone of epidermis (FIG. 100, _££}. They are much larger than the mucus cells and are either round or oval in shape. The nucleus lies in the middle and is surrounded by homogenous ·~ontents. In large club cells the contents are vacuolated. The cells are stained pink with eosin and acid fuchsin; (4) the cutaneous sensory cells or end-buds (FIG. 95,) which are pear-shaped groups of cells similar to those seen in Loricariidae. The cells(~) are elongated with their nuclei lying in their broad base and project slightly out of the epidermis. They are situated on papillae of corium (.£J2.}; (5) the fibroblasts, which are present in the basal layers, (FIG. 94, fb}. No granulated cells are present in the epidermis either of Callichthys callichthys or Callichthys pectoralis. • I I I 79 A distinct basement membrane (FIGS. 94, 100, bm) con- sisting of connective tissue fibres running parallel to the basal border of the epidermis with or without a few connective tissue cells separates the epidermis and cerium. The corium as in Loricariidae consists of two layers, the outer and the inner. The outer layer (FIG. 100, .2.2.) forms a loose network of fibres specially on the body, and bundles of such fibres run vertically towards the underlying scute and penetrate its substance. The blood vessels and the pigment cells (FIG. 94, .12.£) lie immediately below the basement membrane, and there are a few connective tissue (ctc) cells. The inner layer (FIG~ 94, ci) of the cerium consists of bundles of connective tissue fibres running horizontally. The ventral skin of Callichthys callichthys has villi or projections of the outer layer of corium into the epidermis and on some of these villi the cutaneous sensory organs are situated. Similar villi of cerium have been described by HERTWIG (18?6) in Callichthys longifilis where he mentions "Ossification of ventral skin is lacking, surface of cutis is only covered with small villiform papillae". Such villiform papillae are present in most of the naked Siluroidea. WRIGHT (18 8 '4 ) mentions their presence in .Amiurus and VAILLANT and PETI'IT (1898) in Synodontis. .HERTWIG (18?6) and VAILLANT {1895-96) had examined specimens in which the epidermis had been destroyed and the dermal papillae appeared predominant. 80 VAILLANT and . PETTIT (1898). in a subsequent paper remark that "what concerns the difference of aspects between the previous and new specimens of Synodontis schalli is explained by the disappearance of epithelium in the former which left free the derm columns". But tlle structure described by HERTWIG (1876) had not so far been explained and his description is quoted as such by LEEGE (1922). The appearance of the ventral skin of Callichthys longitils, is therefore, not peculiar in any respect as such papillae of corium projecting into the epi- dermis are met with in other Siluroidea as well. In Callichthys pectoralis, however, the outer layer of the corium projects only at those places into the epidermis where the end buds are situated. Dermal Skeleton Structure of the scutes. There are no ossifications on the ventral skin of any of these specieso The body is armoured with two rows of scutes on each side. The scutes meet neither on the ventral nor on the dorsal sur- face. Dorsally the skin between the dorsal and the adipose fin is naked to half its length, and in the posterior half are small oval-shaped median scutes which extend up to the base of the adipose fin. Laterally the scutes extend up to the line of insertion of the pectoral and the pelvic fins. The body scutes form an interlocking arrangement and each covers the anterior border of the other. AGASSIZ (1833-43) describes and figures the scute of Callichthys miles as consisting of an inferior layer of bony tissue with Haversian canals and a superior layer of enamel as in Ganoids. QUEKETT (1855) describes the structure of the bones of the cranium., scale, dorsal and pectoral spines of a Callichthys as consisting of "upper and lower layers, composed of a series of laminae having lacunae between them, whilst the central or diploic portion is cancellous, being permeated by large canals which traverse the compact layers at right angles." He neither described the structure of the dermal dent icles nor noticed the difference in the texture of the different layers of the scute. HERTWIG (1876} has described the structure of the scute and of the denticles in Callichthys longifilis and Callichthys sp., but his interpretation of the upper layer of the scute, which he describes only in c. longifilis and not in Callichthys sp., is open to objection as he investigated specimen~ in which the epidermis was lacking, and he therefore could not study the correlation of the integument and the upper layer of the acute. In Callichthys callichthys the two rows of scutes lie obliquely and meet in the middle line of the body. The acute is more or less rectangular (FIG. 97) in shape and is narrowed at its one end which fits into an articular facet on the ventral ridge of the acute of the other row so as to form an interlocking 82 arrangement. The ventral ridge nuns for almost three-fourths of the length of the scute on its under surface {FIG. 96, vf'). The scute is thickest at its anterior end and tapers towards the posterior edge, its upper surface is divided into two well- marked regions, an anterior and a posterior. The anterior field {FIG. 9?, bsa), being covered by the posterior rield of the other scute, is devoid of any denticles or ornamentation. The posterior field (FIG. 9?, bsp) is ornamented with a net- work of well-defined ridges. On the posterior border (FIG. 98, d) of the scute is a single layer of close set movable denti oles. A few denticles are also found on the surface of the scute, especially towards the short borders if the epidermis is not torn off (FIG. 99, ·d). The scute consists of: {A) an inferior basal layer of lamellated isopedine (FIG. 102, is), the lam.ellae running parallml to the base of the scute with bone cells interspersed between them. The lam9llae towards the anterior end of the scute turn upwards and are prolonged, over the upper surface of the anterior fiefld of the scute, (FIG. 102, bsa), and those on the lower surface are deflected to form the ventral ridge (!.!:). The bone cells of Callichthys have been described by Q,UEKETT ( 1855) as quadrate. Their shape .and form depends on the angle of the section and are best seen in ground sections. 83 They are either oval, quadrate or polygonal (FmG. 105, be) with ramifying oanaliculi which show a tendency to spread in a direction parallel to the lamellae of isopedine so that the texture of the soute appears to consist of regular lamellae of isopedine alternating with layers of bony corpuscles and their branching canaliculii, (FIG. 105, .£.££); (B) the middle vascular layer (FIG. 102, ho) of true Haver- sian canals with concentric bony larnellae (FIG. 101, ho). The canals run in various directions (FIG. 98, 1!.£), receive the canals from the pediments of the denticles and spread in a network in the ventral ridge (FIG. 102, ho). This layer is rich in bone cells; (C) the upper layer, which (FIGS. 99, 100, 101, 102, ~) is homogenous and hyaline in structure and lies over the un- covered part of the scute. It forms the superficial orna- mentation of well-defined ridges separated by deep grooves or depressions which are, in life, filled with blood vessels and connective tissue fibres and communicate directly with the middle vascular layer, (FIG. 102, ho). This upper layer (FIG. 101, .!!l) lies in immediate contact w:il.th the middle bony layer but is sharply defined from it by the absence of bone cells. The bone cells (FIG. 101, ho) lie immediately beneath the upper layer and spread out their canaliouli but do not penetrate into its substance. l . 84 In the substance of the upper layer besides the large grooves containing the blood vessels, run numerous vertical tubes (FIGS. 100, 101, 1!) which start from the upper border, frequently bifurcate into finer branches and end in the bony substance of the acute. AGASSIZ (1833-43) describes this layer as enamel ("l'e'mail") and suggests that bone towards the surface loses gradually its bone corpuscles and becomes trans- parent and brittle, and thus forms a homogenous layer which is in contact with the deeper lying true bone tissue, and which "must be distinguished from the true bone as enamel". That the upper layer in Callichthyidae is not enamel is definitely shown by the action of the acids which do not dissolve it. For the same reason it is not ganoin. .HERTWIG (1876) describes the upper layer in Callichthys longifilis as homogenous sub- stance with "very dense fine vertical tubes descending from the surface to the middle lay~r, branching and forming anasto- moses among themselves. They resemble tooth bone or bone tubes ("Zahnbein-oder knochenrohrchen") and are in contact with th_e processes of bone corpuscles of the middle layer. As the surface of the scale is covered by a thin connective tissue layer which is rich in cells one would expect that from these cells processes would penetrate into the tubes". He, therefore, regards this layer as "modified bone substance with tooth bone tubes ("Zahnbein¥ohrchen") as they have been found and described in the bones of several fish genera". The 85 vertical tubes in the upper_ layer of the scute of Callichthys are likely to be mistaken for the dentinal or longitudinal tubes described by WILLI.A1JSON (1849), HEB~NIG (18?9) and NICKERSON (1893) in Lepidosteus. STEWART (1900) describes similar tubes in Fistularia serrata as passing from the .. sur- face of the bone into the matrix. GOODRICH (1913) is not convinced that the tubes described by STEWART are homologous with the lepidostoid tubes and asserts that so far he has been unable to "discover the lepidostoid structure either in the scale or in the skeleton of any living or extinct species of Teleostei". The longitudinal tubes in Lepidosteus as des- cribed by GOODRICH (190? and 1913) "converge to,wards the cen- tral or the oldest region of the scale and many of them j · penetrate to quite near the ganoin. As shown by HERTWIG 118?9) and NICKERSON (1893) each tubule belongs to one cell which lies on the surface of the scale and sends a long pro- cess down the tubule" •••• "The tubules branch as a rule, only at their inner end and pass outwards to the surface". In ·callichthys callichthys, however, the tubes do not contain any protoplasmic prolongations of cells as was sugges·ced by HERTWIG (1876), but contain bundles of connective tissue fibres which penetrate the substance of the scute (FIG. 100, vt}. By staining with WEIGERT's Resorcin - Fuchsin, which stains the scute bluish pink and connective tissue fibres light pink, picro-indigo-carmin, which stains the connective '.1. 86 tissue fibres deep blue and the substance of the upper layer light blue, and iron haematoxylin, counterstained by VANGIESON, the course of these fibres can well be traced. The connective tissue tibres of the outer layer of the cerium (FIG. 100, co) run vertically, enter the superficial layer and run down to merge into the substance of the middle layer of the scute. Similar bundles of fibres from the outer layer of cerium have been shown by ROTH (1920) to enter the scale border of Gasterosteus aculeatus, though he has neither figured nor described the course of these fibres in the substance of the scute, and describes them as "strong bundles of fibres of the cerium joining the scale border, and also join the other parts on the surface of the scale and make a strong connection with the skin". WILLI.AW.SON (1851) describes and figures in the uppermost tissue of the scale of Ostracion a "superficial system of tubules which open at the surface of the scale", and remarks that "they are not formed by any prolongations of the cell or cell nuclei as no such cells exist but are the result of small apposite apertures left out in each successive layer as -it is added to the preceding one". ROSEN (1913), however, remarks that the canals in Ostracion "are for the blood vessels which pass from the interior to the exterior stratum of the corium". In Callichthys, as the present investigations show, the blood vessels are carried by the 87 Haversian canals which open to the exterior in the deep grooves in the upper layer, while the connective tissue fibres of the outer layer of corium, when the upper layer is secreted by the cells of the corium, become enclosed within it and per- sist as vertical tubes. The upper laye~ in Callichthys, therefore, is neither enamell nor ganoin2 as it is not dissolved by acids, and is not comparable with the cosmine3 of Ganoid scale as it does not contain any cosmine tubes. It is formed by the secretion of the cells of the outer layer of the corium and may be distin- guished as the "osteoid tissue" a term applied by STUDNICKA (1906, b) to the homogenous superficial layer of "cell-free (1) ( 2) (3} ''Enamel is the hardest of all tissues met with in the animal body and at the same time the poorest in organic matter. It is to be regarded as an inorganic substance com- posed of lime salts which have been formed and deposited in particular patterns under the influence of organic tissues, which have themselves disappeared during its formation". (TOMES, 1923). Enamel treated with acid disappears. The term ganoin has been used by WILLIAMSON {1849) for the enamel-like substance of the scale of Lepidosteus. NICKER- SON (1893) asserts that the "outer scale layer in Lepidos- teus is secreted not by epidermis but by cells of dermal origin. Hence it is not enamel in the modern sense of the term~ but may better be known by the name of ganoin, the term introduced by WILLI.AMSON and recently revived by KLAATSCH (1890)". WILLI.AMSON (1849) applied the term. cosmine to a tissue which is more dense in its structure than the true bone, "containing no lacunae, but is always furnished with some arrangement or other of minute branching tubuli". GOODRICH (1907, 1913) defines cosmine · as a tissue with canaliculi like those of dentine, but has restricted its use to a tissue consisting of "pulp cavities or vascular network giving rise to canalicu.li" as in Megalichthys. bone tissue" of Teleosten scales. STUDNICKA. (1906, b) has demonstrated the presence of "numerous penetrating Sharpey's fibres in Osteoid or cell-free bone tissue" of many Teleostei such as Belone, Am.iurus, Lophius, and Orthogoriscus and regards them "to play the same role as the perpendicular :fibres in the corium or the skin". The osteoid layer in Oallichthys shows fine transverse striations which are evidently zones of growth, and, as they run parallel to the outer surface, the addition of new layers has taken place from the external surface. It is thickest in the middle of the scute and gradually diminishes towards the border and ends at the base of the pediment of the denticle at the posterior border of the scute (FIG. 102, .!!¥_). In its structure and staining properties the osteoid layer, therefore, differs remarkably from the other layers of the scute: (1) it does not possess any bone cells; · (2) it is not la.mellated but is homogenous and shows striations; (3) it is penetrated by vertical tubes which Contain prolonga~ tions of .the connective tissue fibres and (4} it stains light blue with picro-indigo carmine while rest of the scute is stained deep bluish green, and with Delafield haematoxylin it takes a deeper stain than t4e other layers. Connective tissue f1bres also enter the acute on its anterior and posterior border and merge into the lamellae or isopedine in the same manner as in Plecostomus. 89 Structure of the denticle The denticles (FIG. 99, d) on the surface of the scute lie embedded in the epidermis with the enamel tips projecting out, and are directed backwards owing to the elevated anterior border of the pediment. Those on the posterior border of the scute are larger than those on the upper surface, vary in size from 0.5 mm. to O.? mm. in length and 0.2 mm. in diameter. Their pediments stand as ·distinct elevations and give the border a dented appearance when t _he denticles have fallen off (FIG. 97, hsp). The denticle is pointed at the tip and broad at the base, and has the structure of a tooth, consisting of hard dentine, enclosing a pulp cavity, an enamel cap and pediment. The enamel is yellowish brown, and disappears completely under the action of acid. It is not confined to the tip alone but spreads in very thin layer over the surface of the dentine for about one-third of its length. The enamel tip is not so long as in Loricariidae. The enamel under the action of acid shows concentric rings as in Plecostomus (FIG. 108, er). The pulp cavity is continued into the tip of the denticle to one- third of its length and gives 3 to 5 branches. Nasmyth's membrane separates off .when the enamel has disappeared under the action of the acid (FIG. 109, ,!!E!). Excepting for the branches of the pulp cavity the enamel contains no other tubes, (FIG. 10?, ~). 90 The body of the denticle {FIGS. 10?, 108, de) consists of hard dentine in which no trace of dentine tubes can be discerned. Unlike Loricariidae the dentine consists of homo- genous calcified tissue. HERTWIG {18?6) describes a similar structure of dentine in Callichthys longifilis. It has been possible to grind transverse sections of the denticles by the dry canada balsam method and it has been found that the dentine has a connective tissue-like fabrillar structure (FIG. 110, de). The fibrillae {FIG. 111, f) run parallel to the pulp cavity and continue down as non-calcified connective tissue fibres {FIG. 111, ~) which join the denticle to its pediment. Beyond the connecting area the fibres are further prolonged into the su_bstance of the pediment. The course of the fibrillae is traceable up to the end of dentine at the tip of the denticle and are not visible in the substance of the enamel. The substance of the dentine is also traversed by branches from the pulp cavity but there is no separate system of vascular canals. The -pediment {FIG. 99, 12.) is a circular ring-like bony structure which lies embedded in the substance of the scute. The base of the denticle is not prolonged into the pediment, that is to say, the denticle is not socketed. The description of the mode of attachment of the denticle ~o its pediment so far available is that given by HERTVVIG (18?6) who describes in . ' 91 c. longifilis that "the base of the denticle joins ·t;he very little protruding bone-ring with which it is connected by a ring ribbon, without entering into its cavity". The ground as well as the decalcified sections of the denticles, however, show that the attachment is not by a ring ribbon, but is dis- tinctly by means of connective tissue fibres which attach the base of the denticle to the external and the internal wall of its pediment and merge into its substance, (FIG. 99, ~). The denticles are freely movable, the capacity of their movements, which depends on the extensibility of the connecting fibres is greater than in Plecostomus as they are not socketed. In their natural condition th6'J lie inclined backwards. Appli- cation of any force or rough handling breaks them off their pediments. In ground sections it is difficult to keep the denticles intact. It is probably f'or this reason that QUEKETT (1855) has not described the structure of any denticle in his description of a Callichthys. Fins. The spines of the fins are studded with well-developed movable denticles (FIG. 112, d) as thickly as in Plecostomus, yet their integument is of the same structure as that of the body. No reduction in the elements of epidermis has been brought about by the thick armature of the denticles. The epidermis consists of: (1) epithelial cells with a distinct basal layer of columnar cells; (2) mucus cells in the upper .. j 92 layers, opening to the exterior by short necks; (3) club cells, which are large, oval or rounded, lie in single layer in the middle region of the epidermis; (4) sensory end-buds which are of the same shape and structure as on the body; and (5) fibroblasts, which are present in abundance in the lower layers. The sac-cells (Kornerdrusen) are altogether absent. The corium shows two distinct layers, an outer, fanning loose network of fibres, which extend towards the underlying bony plate and join its border, but do not penetrate into its substance; and an inner, consisting of bundles of connective tissue fibres running in the same plane as the border of the bony plate forming the spine. The bony cylinder (FIG. 112) of the spine lies in the inner layer of the cerium and encloses e.o,nnec"td v e a central cavity (.££!) which containsitissue fibres and cells, fat and blood vessels. It consists of a basal lamellated layer of isopedine (is), a middle layer of Haversian canals, and an upper layer formed by the pediments of the denticles (.~). The Haversian canals (he) run internally, open into the central cavity and receive at their distal end the canals from the cavities of the pediments (~). Sometimes they open to the exterior on the surface of the bony cylinder. HERTWIG (1876) points out that in Callichthyidae the first ra:y in "pectoral and dorsal fin consists of hard spine ending in long and sharp point, its surface is smoth and lacks any teeth". ' \ 93 But the study of the pectoral and dorsal spines of all the species of Callichthyidae does not confirm his statement, as on the distal or outer surface of the spine are the elevated bony pediments bearing the denticles. There is no super- ficial hyaline layer as seen on the scutes. The fin spine, therefore, consists of true bony texture, with numerous bone cells (FIG. 104, be), which are quadrate, oval or round in shape. In the lamellated isopedine their canaliculi run in the same direction as the lamellae, while in the central region, specially below the pediments and the serrations, they possess canaliculi which ramify in all directions and spread like a spider's web. (FIG. 104, bee). The denticles are present on the under surface of all the fin rays of the pectoral and ventral fins in 2 to 3 longi- tudinal rows but are lacking on their upper surface. The pectoral spine (FIG. 112), the first undivided ray of the pelvic fin and the dorsal spine are covered all over their surface with denticles, which have the same structure as those on the s~utes, and are not socketed. The pediments (FIG. 112, 12,) are slightly elevated from the surface of the spine and lie embedded in the outer layer of the corium. The denticles are inclined towards the free tip of the spine, and in a 140.mm. long Callichthys callichthys they va-ry in size from 0.7 mm. to 0.9 mm. in length and o.2 to 0 . 3 mm. in diameter at the base. 94 On the pectoral spine, besides the true denticles, there are serrations on the inner border (FIG. · 113, ~). They make their first appearance in this family and are not found in any member of Loricariidae but are present in all the other genera of Oallichthyidae and also in families Doradidae and Bagridae. They do not possess the same structure as the true denticles but are composed of y._y lamellae (FIG. 113, !) with bone cells possessing ramifying oanaliouli, without any enamel cap and without a pulp cavity. The serrations decrease in size towards the distal end of the spine and, like the denticles, are turned towards its free end and are firmly anchylosed to the underlying plate of bone. It is to be noted that the serrations have not got the fine radiating canaliculi such as are seen in the serrations of other Siluroidea. On the inner bordel". of the spine the Haversian canals do not f''orm any defi- nite layer and the lamellae of isopedine are continued into the substance of the serrations (FIG. 113, ~). In between these serrations run vascular canals which at one end open to the exterior in the depressions between the serrations and at the other end into the central cavity. Ho:elosternum GILL Ho:12losternum littorale RANCK., (Syn. Callichthzs littoralis) is another member of the family Callichthyidae . The body is covered on each side with two rows of scutes, with small diamond I 95 shaped plates between them along the middle of tbe back. The scutes are larger anteriorly than they are towards the posterior part of the body and exhibit the same interlocking arrangement as seen in Callichthys. Integument. The ventral skin is not raised into villii (FIG. 114~, and the epidermis(~) consists of : (1) eight to ten layers of epithelial cells, with a basal layer or oval cells (FIG. 115, epb), two superficial layers of flattened cells, and the remaining layers of more or less round, oval, oblong or polygonal cells, with a large nucleus lying in the middle of the cell; (2) the mucus cells (.!!!£), which have the typical goblet shape with deeply staining network like contents and a flattened nucleus, lying at the basal wall. They are present in the middle layers but not in such an abundance as they are in the upper layers where they open to the exterior by a Vii.de opening, (FIG. 115, .f!!:Q,); (3) the club cells which lie in a single layer in the middle region of the epidermis, are oval or round in shape, mostly with vacuolated contents, and with a large nucleus lying in the middle of the cell (FIG. 115, .Q.£); (4) cutaneous sensory cells or the end buds (FIG. 116), which are borne on papillae of the coriu.m. as in Callichthys; (5) fibroblasts, which are present in the lower layers (FIG. 115, :f'b) • - 96 The sac cells or granulated cells are totally lacking in this species. The corium is separated fDom the epidermis by a basement membrane (FIG. 115, mb) and consists, as usual, of two layers, the outer and the inner. On the ventral skin (FIG. 114, ~), which has no scutes the distinction between the two layers is not very sharp. On the body the outer layer, as in Callich- thys, consists of a loose network of fibres which join the border of the scute and penetrate into its substance, and pro- jects into the epidermis at places where sensory buds are located. Dermal Skeleton Structure of the Scutes. The abdomen and the ventral surface of the head are naked. The body scute (FIG. 11?), has one short end which fits in the articular facet on the ventral ridge of the other row to form an interlocking arrange- ment. The upper surface of the scute as in Callichthys is divisibJ.e into t·wo areas, the anterior and the posterior. The anterior field (FIG. 11?, hsa~ iv covered by the posterior border of the preceding scute~ is devoid of any denticles and has a ventral ridge (FIG. 122, .n,:) • The posterior field ·( FIG. 11?, bsp) has an almost shining surface, without any well- defined ridges, and does not show the same ornamentation as 97 in Callichthys. The grooves separating the ridges are small and the ridges are consequently smoothed into a level surface. The posterior border has a single row of close set denticles (d}, which are also present on the rest of the surface of the scute but are so loosely attached as to be rubbed off by handling, leaving no apparent trace of their existence, except tiny pediments, which frequently fall off. On the short sides of the scute, thate is the parts which are least handled, the denticles are found (FIG. 120, d'}. The scute as in Callichthys, consists of three layers, (1) a basal layer of lam.ellated isopedine (FIG. 122, is}, containing bone cells with branching canaliculi; (2) the middle vascular layer (FIG. 122, .!!£} of true Haversian canals which are surrounded by concentric lamellae of bony material containing bone cells (FIG. 119, ho'); and (3) the upper homogenous layer of osteoid tissue which covers the exposed portion of the scute, is thickest in the middle and tapers towards the posterior edge where it ends at the base of the pediment ~f the denticle (FIG. 122, ,gz). This layer is pierced by the Haversian canals which open on its surface, and is alm traversed, as in Callichthys, bl{ numerous vertical tubes which start from the s.irface of the scute and merge into the substance of the underlying bony layers. (FIG. 119, ,!!) • This osteoid layer is without any bone cells and shows 98 striations which run in the same plane as the surface of the scute. It has the same structure and staining properties as in Callichthys. The structure of the denticle The structure of the denticle is the same as in Callich- thys (FIG. 121, d) and consists of enamel cap (~), hard non- tubular dentine (de) which is lined to half or one-third of its length by a thin layer of yellowish brown enamel, and a pulp cavity (pea) which gives two or more branches to the enamel (pcb). At the base it is continued into the cavity of the pediment to which the denticle is attached by fibrous tissue (~) and is not socketed. The pediments (~) are of true bony structure and lie embedded in the upper osteoid layer of the scute and are not elevated from its surface as in Loricariidae. The pediments (~) of the denticles on the posterior border of the scute are slightly elevated and give an indented appearance to the border of the scute. The pectoral spine is studded with denticles and in ~ddition it has as in Callichthys, a serrated internal border (FIG. 123, ~). The denticles have the same structure as those on the scute. Their pulp cavities open into the cavi- ties of the pediments, which join the vascular canals, and the latter open internally into the central cavity of the spine. The serrations have the same structure as in Callichthys and do not possess any radiating canaliculi but are profusely in- vaded by the bone cells. 99 Corydoras, LACEP. Corydoras is another genus belonging to the family Callichthyidae. The body is covered with two rows of inter- locking scutes with a row of small median scutes at the dorsal surface. The study and examination of Corydoras aeneus, GILL., Corydoras kronei, RIBEIRS., Corydoras paleatus, JENY., Corydoras agassizii, STEIND., and Corydoras elegans, STEIND., leaves no doubt that Callichthys sp., described by HERTWIG (1876) is not a species of Callichthys but of Corydoras. It is in the latter genus that one meets with small bony plate- lets with one or more movably articulated denticles lying embedded in the ventral skin. HERTWIG's (1876) description of his CAllichthys sp., that in the "ventral skin are bone platelets just seen with the naked eye ••• and at great dis- tance from one another" conforms in all respects to the struc- ture of all the other members of the genus · Corydoras studied Integument. HERTWIG (1876) has not given any account of the structure of the integument of the species as its epi- dermis was torn off. RAUTHER (190?) describes the structure of the integument of Callichthys punctatus, BLOCH., which .is now recognised as a species of Corydoras - Corydoras melanis- ~' REGAN, 1912). From RAUTHER's description and from the study of the integument of all the other available species it has been possible to determine that the epidermis of Corydoras 100 differs remarkably from that of Callich thys and Hoplosternu.m in possessing sac-cells or granulated cells (Kornerdrusen"). Presence of these cells may be regarded as a definite generic character of Corydoras. The epidermis consists of the follow- ing elements: (I) Epithelial cells (FIGS. 124, 125) which lie between the gland cells of the epidermis. The cells of the upper layer are flattened, while those of the middle layer (.~.£.£) are oval or polygonal and of basal layer oblong (FIG.125, eph) in shape; (II) the mucus cells (.!!!£} which are pear-shaped, either empty or with network like contents, deeply stained with mucin stains, and with flat nucleus lying at their base. They lie in the upper layers of the epidermis and open to the ex- terior; (III) the club cells (.££), which are oval, round or oblong in shape, larger than the mucus cells and lie in the middle region of the epidermis. They lie in single layer in a 22 mm. long specimen of Corydoras aeneus, and in two layers on the ventro-lateral skin of a 4? mm. long specimen of the same spacies. In other species they lie in one or two layers. Their nucleus (.!!£) lies in the middle of the cell and is sur- rounded by slightly granulated contents. In large cells the contents are vacuolated. They stain pink with eosin and acid fuchsin and do not open to the exterior; (IV) the sac.cells or granulated cells ("Kornerdrusen") (FIGS. 124, 125, .s.2,} which lie in the middle region of the epidermis along with the club 101 cells, and are almost of the same shape and size except that their distal ends are prolonged into a long neck which opens to the exterior. The body of the sac-cell is filled with granular contents, the granules in some are very fine while in others they are large and coarse. The granular contents when the cell opens, accumulate in the middle of the cell (FIG. 125, .Bf) leaving a clear margin around the periphery. The nucleus is flat and lies at the base of the cell. The granules are stained black by iron haematoxylin but do not take any stain with Delafield haematoxylin. Staining with iron haematoxylin and counterstaining with Rubin Sin picrate of ammonia stains their contents brilliant red and thus makes the sac cells easily distinguishable from the club cells which are stained light pink; (V} the cutaneous sensory cells or end-buds, which are similar to those found in other members of the family, are groups of 5 or more elongated cells, and occupy whole thickness of epidermis (FIG. 124, nb). - They rest on a small papilla of corium; (VI) the fibroblasts which are found in large numbers in the basal layer (FIGS. 125, fb), A distinct basement membrane (FIGS. 124, 125, .£!!!) separ- ates the cerium from the epidermis. The corium consists of an outer and inner layer, the outer forms a network of loose connective tissue fibres, and on the ventral surface some of these fibres run vertically and penetrate the substance of t he cells, and are almost of the same shape and size except that their distal ends are prolonged into a long neck which opens to the exterior. The body of the sac-cell is filled with granular contents, the granules in some are very fine while in others they are large and coarse. The granular contents when the cell opens, accumulate in the middle of the cell (FIG. 125, ,B!) leaving a clear margin around the periphery. The nucleus is flat and lies at the base of the cell. The granules are stained black by iron haematoxylin but do not take any stain with Delafield haematoxylin. Staining with iron haematoxylin and counterstaining with Rubin Sin picrate of ammonia stains their contents brilliant red and thus .makes the sac cells easily distinguishable from the club cells which are stained light pink; (V) the cutaneous sensory cells or end-buds, which are similar to those found in other members of the family, are groups of 5 or more elongated cells, and occupy whole thickness of epidermis (FIG. 124, .a!2,). They rest on a small papilla of cerium; (VI) the fibroblasts which are found in large numbers in the basal layer (FIGS. 125, fb), A distinct basement membrane (FIGS. 124, 125, bm) separ- ates the corium from the epidermis. The corium consists of an outer and inner layer, the outer forms a network of loose connective tissue fibres, and on the ventral surface some of these fibres run vertically and penetrate the substance of the 102 scute. They pass from the outer to the inner layer of the cerium (FIG. 133, co). Blood vessels and pigment cells lie beneath the basement membrane, and connective tissue cells lie scattered in the network of the fibres. The inner layer of the cerium (FIG. 124, ci) consists of bundles of connective tissue fibres running horizontally and in the same plane as the border of the scute. Ventral grnute s. Tiny bony platelets are present in the ventral skin of all the species studied. In Corydoras aeneus, 22 mm. long the scutes have not yet appeared, in a 38 mm. specimen they have just appeared and lie scattered about at some distance from one another. In 47 mm. and 50 mm. long specimens they are fully developed (FIG. 126, vs). The scutes are irregular in shape, without any unifvrmity in their form or size and lie in between the two layers of the corium (FIG. 133, ~). The anterior end of the scute lies deeper in the corium than t he posterior end, which is raised towards the outer layer of t he cor i um and bears one to five den ticles . In decal.cified sections the scute is seen to be traversed by connective tissue fibres in bundles which penetrate from its surface and pass int o t he inne r laye r s of t he corium (FIGS.133 , 134, ,£!). Some of t hese fibres after emerging from the scut e run vertically, in the corium, while others take a horizontal direction (FIG. 133, ~) . The edges of the scute and its basal layer are also traversed by connective tissue fibres 103 which mingle with those of the cerium at the borders of the scute. The scute thus appears to be composed of a felt-like texture of connective tissue fibres, densified and calcified with a few bone cells. Blood vessels also run through the substance of the scute, enter the cavity of the pediments and supply the pulp cavity of the denticle. homogenous layer in the ventral scute. There is no upper Ventral scutes of Gorydoras kronei (FIG. 128), 62 mm. long, are similar to those of Gorydoras aeneus, but are thicker in structure with Haversian canals and lamellated isopedine. The denticles vary in number from one to three on each scute. 1 In Gorydoras paleatus, 42 mm. in length, (FIG. 131, vs) the ventral scales are few and scattered, lying apart from each other and bearing one, two, or more denticles, They are irregular in shape and have a true bony texture. In .Q2!l- doras trilineatus 35 mm. long (FIG. 132, _!!) the number of the ventral scales is small. They are irregular in shape, lie apart from one another, and each bears from one to two or more denticles. In Corydoras agassizi, 45 mm., (FIG. 12?, vs) the skin is very thickly set with irregular scales, each bearing ( 1) On one scute there is a branching denticle (FIG. 129, d). It has one base, (b), where it is attached to its pediment, and one pulp cavity (pea) which communicates with the cavity of the pediment(~). But the pulp cavity near the middle of the denticle divides into two branches each entering into each division of the denticle, which possesses two yellowish brown enamel capped tips ( e) , in to which the pulp cavity gives two or three branches. -Branching of a denticle is a rare phenomenon and is not a normal process, still it reminds one of the branching so frequently met with in the placoid scales of Elasmobranchs. 104 one, two, or more denticles at its posterior border. In Corydoras elegans, 35 mm., (FIG. 130, vs), the ventral scales are nwnerous, oblong or irregular and each bears only one denticle. Thus the ventral skin of this species differs defi- nitely from all others as the number of the denticles is limi- ted to one only. Freshly developing denticles are seen in between the fully developed ones on the ventral scutes, thus indicating that replacement of the denticles does take place, (FIGS. 126, 130, dd). Ventral scutes of Loricariidae are mentioned in the description of the species of that family in its systematic account, but this practice has not so far been followed in the case of Callichthyidae. In all the species of Corydoras examined, they form a constant feature, but whether they form a specific character for the indentification of species is rather difficult to state as their number and arrangement differs at different periods in the same animal. These scutes should, however, be regarded as a generic character of the genus Corydoras. The body scutes, (FIG. 135), as in the other .members of the family, are rectangular and are produced into a small end at one border to form an interlocking arrangement with the scutes of the second row. The surface exhibits the same two 105 areas as in Callichthys, an anterior (FIG. 139, ~), covered over by the posterior of the preceding scute ~d without any denticles, and a posterior(~), which is beset with small denticles (i!_:) which lie in rows over the whole of the surface and also form a single row at the posterior border of the scute, (FIGS. 135, d, and 139, d). On the ventral surface of the anterior area lies the ventral ridge (FIG. 138, .!!:) , The scute is thickest at the anterior end (FIG. 139, bsaJand tapers to- wards the posterior border to the thickness of the pediment of the denticles (FIGS. 138 and 139, ~). The external surface of the posterior field has no ridges and is smooth and shining. The scute has the same structure as already described ~or Callichthys (page 82 ) except that the upper osteoid layer (not described by HERTWIG, 1876) is thin (FIG. 138, !!z), and is pierced by Haversian canals which open to the exterior, and is traversed by fine vertical tubes which contain the connec- tive tissue fibres of the outer layer of the co rium. The basal layer of la.mellated isopedine (FIG. 138, is) contains bone cells which have branching canaliculi running in the same direction as the lamellae of isopedine. This layer, as seen in ground sections is pierced by fine fibres of connective tissue which in dry condition leave spaces filled with air in the shape of tubules (FIG. 138, lpd). Such tubules, as has already been mentioned in the case of Loricaria 106 cataphracta, were hamed as lepidine tubes by WILLIAMSON, (1849-51), who regarded their function to be "the general nutrition of the scale". In the transverse ground section of the scute of c. aeneus the continuation of these "tubes" with the connective tissue fibres emerging out of the scute can clearly be seen. There is no reliable account of the distribution of the denticles on the scute of Corydoras. They are so loosely attached to their pediments that they fall off even with slight handling. LEEGE (1922) states that in "Callichthys and Corydoras mostly pointed and flatly bent teeth are formed only on the posterior margins of scutes. The proper surface of the shield is free of teeth". HERTWIG, (1876) in his description of Callichthys sp., which for the reasons already stated is to be regarded a species of Corydoras, points out that "in the arrangement of teeth large scales of the side differ from small ones in the tail region: on the posterior margin of the large scale is a single row of small teeth, which are very close to one another, and are hardly raised above the skin surface as they lie almost horizontally in the epidermis and maintain the same direction as the surface of the scale ••••• The arrangement of teeth on the smaller scale of the tail is different as we not only find a single row of densely placed teeth on the posterior convex margin, but also on the rest of the free 10? surface of the scale we can perceive partly small teeth and partly empty bone rings at a greater distance from one another". Examination of very well preserved specimens of half-a- dozen species of Corydoras and over a dozen specimens of c. aeneus of different sizes, leaves no doubt that the denti- cles on the body scutes of Corydoras are uniformly distributed over the whole of the free surface of all the scutes (FIG. 135, d}. In many specimens the denticles are lacking on those parts which are likely to be handled frequently such as the middle parts of the sides of the body. In such cases the denticles are still visible on the upper and lower borders of the scutes, i.e. parts which have not been handled (FIG. 136, d'}. The denticles (d} on the posterior margin of the scute are larger and stouter than those on the body and are not so easily rubbed off. The description given by HERTWIG, as he has himself admitted, is from a specimen in wh.ich "the epidermis is torn off". So it was not possible for him to notice the uniform distribution of these denticles. The d~nticles on the surface of the scute are uniform in size and thickness (FIG. 136, d'}, and do not increase in size antero-posteriorly as they do in Plecostomus. Fresh denticles are perceived at the posterior margin of the scute in the skin fold that connects the scutes together (FIG. 136 and 137, dd}. On the surface of the scute empty pediments sunk in the upper layer are visible but very often the pediments 108 are also torn off wtth the denticles. Nowhere on the surface are there any indications of the replacement of the denticles. The denticles once lost are not replaced and that is another reason why their regular presence has not been noticed by previous investigators. In c. aeneus 22 mm. long (FIG. 137), there are 2 to 4 denticles in each transverse row, and in a 4? mm. specimen (FIG. 136}, the number varies from 4 to 8 in each transverse row on the surface of the scute. The denticles (FIG. 140, d} have the same structure as in Callichthys. On the denticles of the ventral scales of a 22 mm. specimen of c. aeneus the enamel is pale yellow, while in older specimens measuring 47 mm. it is yellowish brown. Formation and growth of the scute in Callichthyidae The only account of the formation of the scute in Callich- thyidae is that given by BALLANTYNE (1930) in Hoplosternu.m littorale. "The bony plates have not made . their appearance in development stage 35. At stage 39, in a 22 mm. larva, the plates are beginning to form down the sides of the body, but as yet do not reach very far up dorsally, or down ventrally. As in the adult, there is a row of denticles along the posterior border ••••• In a transverse section through the middle of the trUiilk: region, the anterior end of one bony plate can be seen lying deeper in the tissue than the posterior end of the plate in front. Along the edge of this plate are numerous little 109 denticles. The plate lies in the thick dermal layer just below the epidermis. As there is a gap in the series of larvae between stages 35 and 39, it is impossible to obtain sections of the earliest stages in the development of the bony · plates". It has not been possible to obtain the earlier stages of any of the Callichthyidae and in the absence of such material the development of the scute cannot be fully investigated. In a 22 mm. specimen of Callichthys sp. sent by Professor GRAHAM KERR, the plates are beginning to form down the sides of the body, but as yet do not reach very far up dorsally or ventr- ally. On the surface of each scute there are 2 to 3 rows of denticles in addition to one row at the posterior border. The scutes on the trunk region have two rows and those on the tail region have two to three rows. The denticles, therefore, are not confined only to the posterior border but are also present on the surface of the young scute. In a 22 mm. Corydoras aeneus the body is covered with two rows of scutes, each of which pos?esses 2 to 4 rows of denticles on its surface (FIG. 137). Fresh denticles develop at the posterior margin and the growth of the scute takes place in a similar manner to that in Plecostomus. That is to say tha scute grows by calci- fication of inner layer of cerium, while the denticles are formed in the outer layer of the corium and get attached to the underlying plate by their pediments. The pediments in young 110 animal are projecting above the level of the scute and are covered over by the outer layer of the cerium. At the earl y stage as in 22 mm. c. aeneus the upper hyaline layer is eith er not yet formed or is very slightly developed {FIG. 140). In older stages the pediments (FIG. 143, R) lie embedded in this upper layer (FIG. 143, .BZ)• These facts indicate that the scutes make their first appearance in the inner layer of th e corium, denticles develop independently in the outer layer, attach themselves to the scute, while fibres of the outer layer also attach themselves to the outer border of the scut e. At this stage the connective tissue cells (FIG. 140, ctc) line the borders of the scute, especially the outer border, where by their activities the upper layer is secreted. The secre - tion of this layer definitely begins after the development o f the denticles as it is indicated even in the adult scale, where, as has been seen, the upper layer terminates at the base of the pediment of the denticle of the posterior margin (FIG. 138, ~). As soon as this layer is secreted the pedi- ments of the denticles together with the vertical fibres of the outer corium get embedded in it and the latter persist i n the fully developed scute as the vertical tubes. The scu te grows throughout life and the seat of growth is at the poste rior margin of the scute. If broken the scute, like the scal es of other Teleostei, possesses the power of regeneration as 111 shown in FIG. 11?, where part of a acute of Hoplosternum littorale has regenerated(~). It is interesting to note that there are denticles not only on the surface of the scute but also there is a single row of close-set denticles at its regenerated posterior.border (FIG. 11?, d'). Development of the Denticles Study of the available early stages of Callichthyidae thus indicates that the denticles do develop as a regular feature on the lateral scute, that they are not socketed and are very loosely attached to their pediments, they easily fall off often with their pediments and leave no sign of their ex- istence. Further there are no indications that replacement takes place so that the denticles which are once lost are not renewed. The adult scutes, therefore, frequently show only one row of denticles at the posterior border and are devoid of any on their surfaces except in young and well-preserved specimens. On the ventral scutes and on the spines of the fins, the presence of freshly developing denticles in between the fully developed ones indicates that they are not permanent forma- tions and that their replacement does take place. It is mainly for this reason that on the ventral soutes and on the spines of fins one rinds a regular and uniform covering of denticles. Such fresh buds of the denticles are yellowish in colour on account of the coloration of their enamel caps 112 and can easily be picked out from the spines (FIGS. 106 and are found in abundance on its under-surface. No account of the development of the denticles of Callichthyidae is available. Stages in the development of the denticles are seen on the ventral scale of c. aeneus, 50 mm. long, (FIG. 141). It develops as a small papilla of external cerium over which the epithelial cells of the pali- sade layer form an ename·l organ, which consists, firstly, of long columnar cells (~), the ameloblasts, and secondly, of flat epithelial cells, lying over the ameloblasts forming the ehcn~ enamel <$$5 Se (~). Towards the base of the dentinal papilla the am.elbblasts pass into the basal layer of the epidermis. It is evident, therefore, that ameloblasts are cells of the basal layer. The papilla lies immediately under the epidermis, with its tip directed backwards, and encloses a pulp cavity (~ea) which is rich in cells. - The cells lining the surface of the pulp cavity are differentiated from the connective tissue cells by their enlarged size as odontoblasts (ob). The dentine (de) appears between the layers of amelo- blasts and odontoblasts and the denticle grows by further formation of the dentine. Its base (FIG. 142, ~) extends towards the underlying plate of bone (FIG. 142, ~) as a thin layer of hyaline tissue, while its tip grows up by the forma- t ion of the ename 1. The greater the denticles become by the 113 formation of the dentine and the enamel the more they grow towards the epidermis and pierce it. No prolongations of odontoblasts ever pierce the dentine and no dentine tubes are seen at any stage of development. The dentine, therefore, from its very inception is a solid non-tubular structure: a homogenous calcification of connective tissue fibres which are prolonged basally and join the underlying plate of bone (FIG. 142, ~). It is after the rupture of the denticle that the formation of the pediment takes place as a hyaline calcifica- tion in the connective tissue prolongation CE.~} of the denti- cle base similar to that seen in Plecostomus. Calcification starts near the base of the denticle and grows towards the scute in the shape of a ring and forms the round pediment. During calcification the connective tissue cells become en- closed as bone cells. The area between the denticle and the pediment does not calcify and forms the zone of attachment consisting of connective tissue fibres. Developing denticles (FIG. 106) picked out from the fin spines s.how that though the den ticle is fully developed with a well-formed enamel (e) covering and has pierced the epi- dermis, yet the pediment is still undeveloped, and below the base of the dentine are seen connective tissue fibres (E.!~J which are prolonged towards the underlying bone. On the ventral scales (FIGS. 126, 130) many stages of the developing 114 denticles (dd) are visible. The old denticles with their - . pediments having been absorbed a big gap is left in the scute and on this gap the fresh denticle germs make their appearance. Dentine is the firstto be formed, enamel appears next, and lastly the pediment makes its appearance. It may be of in- terest to note that the arneloblasts do not totally disappear but persist around the base of ·l;he fully developed denticle and are continuous with the basal layer of the epidermis (FIG. 142, (ab). Family DORADID.AE The DOR.ADIDAE form another family of South American Fresh water Siluroidea. The body is covered by a single series of bony plates on each side. Integument. In Deras dorsalis c. & v. t he epidermis (FIG. 144) has the typical structure possessed by naked skin Siluroidea, and consists of (1) the epithelial cells, with a distinct basal layer of collUDilar cells (epb), and an upper layer of flattened cells, while the cells in the middle layer vary in shape on account of the pressure caused by the gland cells; ( 2) the mucus cells (.!!!£), which are goblet-shaped ·in the upper layer of the epidermis where they open to the ex- terior and are frequently empty. They also lie above the basal and in the middle layers where they are either round or oval and with network like contents. The nucleus lies at the 115 basal wall. The cells stain with such mucin stains as methylene blue, toluidin blue and Delafield haematoxlin; (3) the club cells(.£.£), which lie in 2 to 3 layers and are either round, oval or club-shaped, with their oval end .lying towards the surface and the narrow end towards the base of the epidermis. Their contents are homogenous, stained light pink with eosin or acid fuchsin, with one or two nuclei lying in the middle of the cell. In large cells the contents are vacuolated; (4) the cutaneous sensory organs, which are of two kinds (a} the end-buds, situated on papillae of the cerium which projects into the epidermis: {b) the nerve-hillocks or free "Seitenorgane" which are similar to those found in other members of the sub-order and ~onsist of groups of 3 or more large cells with large nuclei (FIG. 145, !!!), and acido- phil contents, supported at the base by epithelial cells {FIG. 145, .§..£), and lie in a cavity which opens to the exterior, (,2). Nerve bundles join their bases (FIG. 145, !!) ; {.£,) the fibroblasts or connective tissue cells (FIG. 144, fb), which are present in the lower layers. The cerium is separated from the epidermis by a basement membrane {FIG. 144, bm) and as usual consists of two layers, the outer and the inner. The distinction between these two layers is not sharp on the ventral skin, but on the body where the scutes are formed the outer layer forms loose network of \ ' 116 fibres which attach themselves to the scale border and pene- trate its substance slightly. Blood vessels and pigment cells are present under the basement membrane and there are a few connective tissue cells (cto). The inner layer consists of . fibres of connective tissue running in the same plane as the surface of the epidermis. It has a few blood vessels and connective tissue cells, and a few vertical bands run in be- tween the horizontal fibres. In D. dorsalis the body scutes have a central spine which increases in height antero-posteriorly. In D. wedellii 118 mm. in length, the spine is of uniform length from the anterior to the posterior end of the body, and is directed backwards. It is slightly grooved and is sharp pointed at its tip. Its base is broad and firmly anchylosed to the scute. It has neither an enamel cap nor a pulp cavity and is not a true den ti cle, but is c amposed of lam.ell a ted bony texture wit;h numerous bone cells (FIG. 148, L). The lamellae are tra- versed by numerous minute canaliculi (.QB.) which radiate from the border of each lamella to the external border. Some of the bone cells in the spine possess such radiating canaliculi which are all directed towards the external surface (FIG. 150, bee). The scute (FIG. 146) is trapezium shaped, with an anterior convex and a posterior concave border, the anterior border is slightly overlapped by the poste:i;aior border of the preceding 117 scute, and at its posterior margin are two or more small back- wardly directed spines. It consists of an upper layer (FIG. 14?, ~) which is not homogenous or hyaline but is lamellated with bone cells scattered between the lamellae. The lamellae of this layer merge into the lamellae of the spine. This layer is traversed by numerous fine vertical canaliculi (FIG. 149, .£!!), which radiate from the lamellae towards the outer surface of the scute and are abundant in the vicinity of the spine. Here, too, some of the bone cells (FIG. 149, .!?.£.:) have their canaliculi directed towards the outer surface in the same way as the radiating canaliculi. QUEKETT (1855) following WILLIAMSON (1851) describes the upper layer in the plate of Silurus as of "dense bone very like the enamel of a tooth" and calls it the "ganoid layer", and regards the canaliculi as resembling the tubuli of dentine. The minute tubules re- semble dentine tubes, but are the canaliculi of bone cells which have been obliterated either by the addition of successive zones of growth of bony material or have been reduced in size and too flattened to be seen clearly. Such cells with verti- cal canaliculi are seen at a few places in the substance of the spine and in the upper layer of the scute. (FIG. lfiO, bee). The middle layer contains the vascular canals (he), and a canal for the lateral line (llc) and possesses an abundance of bone cells with ramifying canaliculi, branching in all directions. ' . 118 The basal layer is lamellated {FIG. 149, is) with lamellae running parallel to the border of the scute, with numerous bone cells, whose canaliculi spread in the same direction as the lamellae of the isopedine. Minute non-calcified fibres (lpd) penetrate the basal layer at the basal border in the same manner as seen in Loricariidae and Callichthyidae. RAUTHER{l929) thus sums up the characters of this family by saying that the "Doradinae are provided on each side with one series of large armour shields. They carry on the pos- terior region more or less numerous spines. Sometimes one big spine bent towards posterior end is present. But these are not true teeth and are of the same structure as the armour plates. They are formations o~ true bony substance, without an eriamel layer and rich in network of canals". Such bony tubercles are also present on the cubito humeral process of the pectoral girdle and on the cranial bones. The pectoral fin spine has serrations on its internal and external borders. The dorsal spine in species where it is strong is also serrated on its internal border. true bony structure. Family BAGRIDAE The serrations are of In Bagridae and Synodontidae there is no armour. The body is naked, but the head and the posterior part of the 119 throat are covered from above by a large solid bony armour which is divided into several parts and is called the "nuchal shield". It is formed by "a process of the supraoccipi tal and by plates which are expansions of the distal ends of the interneurals of the first three rays of the dorsal fin", REGAN, 1911). In Rita rita an Indian Siluroid belonging to the family Bagridae all the three parts of the nuchal shield have blunt tubercles on the outer surface. Such tubercles are also · present on the surfaces of supraoccipital, posterior part of the frontal and then merge into small ridges over the anterior part of the parietal. They are also present on the pterotic, epiotic, sphenotic and cubito humeral process of the pectoral girdle. Integument. Rita rita has the typical integument of naked Siluroidea, and the epidermis consists of: (I) the ordinary epidermal or epithelial cells. The cells in the upper layer are flat or triangular in shape, with their nucleus lying in the centre, the basal or palisade layer (FIG. 151, eph) consists or oval cells usually of the same size as the ordinary epidermal cell, with their broad base resting on the surface of the basement membrane and a large nucleus in the middle. In between the superficial and basal layer the cells of the epidermis vary in shape and size and may either be spindle-shaped or polygonal, (FIG. 151, ~); (II) the mucus 120 cells are of typical goblet-shape and are not bag-like as seen in Callichthyidae and Loricariidae. They occur in the upper layers of epidermis and appear to be distributed uniformly over the surface of the sldn (FIG. 152, me). They are also present near the basal layer as well as in the middle layers. In the deeper layers the mucus cells are oval or round in shape with their nuclei at the base and with network like con- tents which stain deeply with the mucin stains. As they approach the surface they increase in size, and when they open on the surface they assume the characteristic goblet shape as their base becomes broadened and the apex forms a short neck which opens to the exterior by a wide aperture with well-marked network-like contents and with a flat nucleus lying at the base. When the mucin is expelled the cell becomes empty with the nucleus lying at the base. The mucus cells arise from the basal palisade layer of the epidermis,. become filled up with ' granular deeply staining mucin contents and their further course can be traced up through the layers above, increasing in size and finally opening to the exterior. WRIGHT (1884) observed that the mucus cells are formed by the conversion of "ordinary lower polygonal cells", SCHULZE, F.E. (186?), FOETTINGER (18?6), and FRITSCH (1886) regarded their origin to be from the ordi- nary epidermal cells of the upper and middle layers. LIST (1886) in Cobitis and Torpedo, and REID (1894) in the Eel have 121 already demonstrated their origin from the palisade layer; (III) the club cells (FIG. 152, .£.£) lie in 3 to 4 rows and vary in form, the smaller ones are more or less round or oval, while the large ones are prolonged into a neck which extends towards the basal layer of the epidermis, (FIG. 154, a, b, c). In a specimen measuring 235 mm. in length the epidennis is 0.48 mm. thick, and the largest club cell measures 0.2 mm., while most of them range in length from o.128 to 0.160 mm. in length. VAILLANT and PETTIT (1898) record in the case of Synodontis schalli BLOCH., that the dimensions of club cells are "colossal, some of them indeed are more than i mm. in length ( 500 /A-) and most of them measure 3/10 to 4/10 mm." OXNER ( 1905) has pointed out that size of the "club cells not only of different genera but also in one and the same indi- vidual vary within a wide range". He records in Silurus glanis a height of 0.0?2 to 0.162 mm. and thickness 0.036 to 0.054 mm. In small club cells the contents are homogenous, stained pinkish with eosin and acid fuchsin, contain one large or two small nuclei in various stages of separation from one another and always situated in the centre of the cell. In large club cells the contents become vacuolated, the vacuoles appear- . ing either at one or both ends or all round the cell border. (FIG. 154). Club cells on the surface of the epidermis are seen protruding (FIG. 151, .££). WRIGHT (1884) considered the I.I 122 protruding of the club cells from the surface as "probably due to a defect in the superficial layers of the epidermis and to the action of the hardening reagents". FOETTINGER (18?6), however, followed their actual extrusion in Cyclostomes, and subsequent researches of BEID (1894}, OXNER (1905), NORDQUIST, (1908}, PAWLOWSKY (1911} and others have undoubtedly estab- lished the excretory function or the club cells. The cells come to the surface and are eventually expelled as a whole structure. The other function of the club cells as pointed out by OXN"ER (1905), is that of support or protection for the soft epidermis; (IV} the cutaneous sensory organs which are of two kinds, (a) the end-buds (FIG. 153, (.a£), which are freely distributed on the skin especially on the head and are situ- ated on projections of corium into the epidermis, with bundles of nerves attached to their base. The upper surface of the end bud is level with the external surface of the epidermis: (b) the nerve-hillocks, which are similar to those seen in Deras and in Loricariidae. (V} the fibroblasts (FIG. 151, !!?_) which are found ~n large numbers in the lower layers. The corium is separated from the epidermis by a basement membrane {FIG. 151, J?.!9), and consists of two layers, the outer and the inner, but there is hardly any distinction between the two in the structure of the fibres, except that the outer layer 123 is raised into papillae (FIG. 152, .£:12.) which project into the epidenn.is. The palisade cells of the epidermis are arranged on these papillae in such a manner as to cause the remaining cells to look as if arranged in pockets (FIG. 152). Such papillae have also been described by WRIGHT (1884), in .Amiurus, and by VAILL.ANT and PETTIT (1898) in Synodontis. VAILL.ANT and PETTIT regard the papillae "not as tactile organs but as serving to keep the great epidermal cells upright and perform the nutrition". In Rita rita pigment cells, and blood vessels are carried into these papillae and at places where end buds are located nerve bundles also pass into them to be attached to the sensory groups of cells. So these papillae in addi- tion to serving as support and for nutritive purposes have also a tactile function at those places where sensory groups of cells are situated on them. Pigment cells (FIG. 151, ]2£) and blood vessels lie be- neath the basement membrane and there are also a few scattered connective tissue cells. The inner layer (ci) of the corium is forme~ of lamellated bundles of fibres running parallel to the surface of the epidermis, diverging at places to form cavities and not interwoven but traversed by a few vertical fibres. There are a few connective tissue cells scattered in between the bundles of fibres. The nuchal plate (FIG. 155) consists of bony larrellae forming the basal layer, with bone cells having ramifying 124 canaliculi, which tend to spread in the same direction as the lan:ellae, a middle layer of true Haversian canals {.!!£) and an upper layer of bony lamellae which form the tubercles. The tubercles (FIG. 156) are without any enamel or pulp cavity and have the same structure as the underlying bony layers, The vascular canals start from the base of the plate and open to the exterior in the depressions between the tubercles. There is a clear bony area in the centre of each tubercle (FIG. 156, ..£12) representing the starting point of the growth over which the bony lamellae (1) are laid. In between the successive larnellae of the tubercles are numerous vertical canaliculi, (,£!!), radiating from the inner surface of the plate to its outer surface and spreading out between the successive lamellae, crossing them at right angles similar to those seen in Doras. The regular arrangement of the bony lamellae on each tubercle suggests zones of annual growth as each lamella is separated from the other by a clear and distinct line. These catfishes inhabit the rivers of North India where in winter temperature falls down to 40° to 50oF. and in summer rises to- ?5° to 85° F. and it is possible that these varia- tions in the temperature produce different rates of growth, which fonn a definite pattern on lamellae seen on the tuber- oles. Each zone consists of lamellae with bone cells and an abundance of canaliculi. Whether it is possible to determine the age of such a catfish by the number of zones on the 125 tubercles is a problem that requires further investigation. The nuchal shield lies embedded in the coriurn and its tuber- cles lie immediately beneath . the epidermis but do not penetrate it. When the epidermis is rubbed off they become exposed. QUEKETT (1846) has pointed out that bone cells in the majority of the bones of fishes are not present, their place being occupied by enlarged canaliculi or tubes. In 1859 KOLLICKER also observed that many genera of Teleosts possess "no bone corpuscles". The upper layer in the tube roles of the nuchal plate of Rita rita illustrates the process by which the typical bone cells become modified into canaliculi or tubes by the elimination of the cell body. The upper layer is, therefore, not to be regarded as any special layer be- cause of the presence of radiating canaliculi as it is of the same morphological value as the remaining bony layers. In addition to these canaliculi the lamellae of the tubercles also contain ordinary bone cells which have their canaliculi directed towards the outer surface of the tubercle and lie in the same plane as the radiating canaliculi. The cubito humeral process is made up of an inferior bony layer with lamellae running in the same plane as the border of the process and continued above into a vascular layer of Haversian canals and an upper layer of bony lamellae forming the tubercles as in the nuchal plate. The tubercles have the same structure as in the nuchal plate with bony 126 lamellae containing bone cells and traversed by numerous radiating canaliculi. The pectoral spine of Rita rita is ornamented with small ridges on its surface and in addition to these it has two rows of sharp pointed serrations at its inner and outer border. The serrations (FIG. 157, sr) lie in shallow grooves and are triangular in their outline with a sharp pointed tip which is directed towards the free extremity of the spine. The spine has a central cavity filled with connective tissue and fat and the ridges on its surface are produced as a result of the deflection of bony lam3llae and are profusely inter- spersed with minute canaliculi similar to those seen in the tubercles of the nuchal plate and which can in some cases be traced as belonging to bone cells. At the inner and the outer border the larnellae of these ridges are directly con- tinuous with the lam3llae of the serrations (1) which have the same structure as the tubercles on the nuchal plate with bone cells in the middle area and numerous canaliculi in their lamellae! There is an elaborate network of canals (FIG. 157, .!!.£) which open to the exterior on the surface of the spine by many apertures, and internally into the central cavity. 127 DISCUSSION (a) Integument. The epidermis in the Sub-Order Siluroidea shows remark- able diversity of f©:l!'m, -in the different families and in eome cases its glandular elements possess specific generic characters. The gland cells in the epidermis of Siluroidea are of three kinds, firstly, mucus cells, secondly, club cells, and thirdly, sac cells. Mueus Cells: Mucus cells or goblet cells or "Becherzellen" of SCHULZE, F.E. (1867) and "Schleimzellen" of Ll!."YDIG (1851) or "Cellulae muciparae" of PAWLOWSKY (1911) and others, are of uniform occurrence in all the members of Siluroidea, and attain their highest degree of development in some Loricariidae. They belong to -the type of mucus cells described by LIST {1886) as "Becherzellen" and by PAWLOWSKY (1911) as "Cellula calcy- formes" being beaker, goblet or bag-shaped with the nucleus . always lying towards the periphery at the base. The second kind of mucus cells "round or spherical in shape with central nucleus" described by LIST (1886) as "Leydig cells" are lacking 128 in Siluroidea. The mucus .cells stain blue violet with Delafield haematoxylin counterstained by eosin; violet with thionin toluidin blue and methylene blue. In the lower layers of epidermis these cells are round or oval in shape with peri- pheral nucleus and either homogenous contents or with loose network with mucus granules lying in the meshes of the network. As they pass towards the superficial layers they increase in size, their base broadens, and apex forms a short or long neck which opens to the exterior. When the mucus is expelled the cells are seen as empty with the nucleus still lying at the base. In same of the Loricariidae such as Lipopterichthys, Xenocara and Arges they have exceptionally long necks. In Arges the base of the cell lies just above the palisade layer and the ne1ck is prolonged through half the thickness of the epidermis to open to the exterior. Such bag-like mucus cells have also been described by LIST (1886) in Cohi tis fossilis and by various authors in other fishes. In the other members of Siluroidea the mucus cells are either oval or goblet shape with a broad base and short neck. Development of large mucus cells in Loricariidae compen- sates to &lme extent the absence of other gland cells and pro- duces a thick slimy covering on the skin. 129 Club Cells: Club or clavate cells of WRIGHT (1884), "Kolben" cells or "Kolbenformige Gebilde" of SCHULTZE, M.J.S. (1861), and of others have received an exhaustice investigation by OXNER (1905). RAUTBER (190?) pointed out the absence of club cells in the adult Loricariidae but noticed their presence in young Plecostomus. The present investigations, however, show that the so-called club cells in young Plecostomus are not in any way similar to the club cells of Physostomi but belong to the category of sac or granulated cells. It may therefore be stated that the club cells are totally absent in the family Loricariidae at all stages and to the statement of 0:x::N"W~ ( 1905) that "within the order of Teleostei club cells occur only in Physostomi, excepting the family Salmonidae in which club cells are lacking entirely", the family Loricariidae may be added as forming another exception. RAUTHER (190?) has stated that in Callichthyidae the club cells are confined to one layer. Study of adult specimens of Callichthys and Corydoras, however, shows that in Callichthys pectoralis ?4 mm. long and Corydoras aeneus 4? mm long the club cells lie in 2 to 3 layers on the ventral skin and in one layer on the body skin. In Corydoras aeneus 22 mm. long and in Hoplosternum littorale they lie in one layer on the body as well as on the ventral skin. 130 On the pectoral spine of Callich thyidae where the amour of denticles is as thick as in Loricariidae the club cells are present in as much abundance as ·in naked Siluroidea. In the naked-skin sub-family Argiinae of the Loricariidae, in which there is no body armour, there is a total absence of these club cells. There is, therefore, no evidence to support RAUTHER's statement (1907) that there is in Siluroidea, a correlation between the frequency of the club cells and the skin skeleton. The presence of club cells, a characteristic feature of Physostomi, in Callichthyidae is strong positive evidence~~ show their Physosmomian character and indicate their affinity to the naked Siluroidea. Absence of such cells in Loricariidae, however, need not be considered as having any phylogenetioal value as the club cells are lacking in Salmonidae as well. But their absence in the family Loricariidae affords an in- teresting line of study for the relationship of its sub- families as, for instance, in Argiinae, where though the body is naked and the body armour absent, yet the epidermis has the typical Loricariid stmucture and totally lacks the club cells. It may therefore be added to the characters already enumerated by REGAN (1903} that the structure of the skin of Argiinae gives further evidence to show that the nakedness of the body is a secondary condition. 131 Sac Cells or Granulated Cells: RAurHER ( 190?} pointe·d out the presence of granulated cells ( ''Kornerdru.sen"} in the epidermis of Callichthys punctatus BLOCH., which is now recognised as Corydoras melan- istius, REG. Such cells have also been described by KWIET- NIEWSKI (1905} in Acanthicus vulgaris, by BYKOWSKY and NUSBAUM (1905) in Fierasfer, cuv., by STUDNICKA (1906) in Lepadogaster, by NUSBAUIJ u. KULCZYCKI (1906} in Fierasfer dentatus EM., and by HASE (1911} in cyclopterus lumpus L. STUDNICKA (1906) describes them as a new kind under the name of "Sackformige serose Driisen". PAWLOWSICY ( 190?} describes them under the same name in Trachinus draco. HASE (1911) describes them as "Offene Kolben" and regards them as "further developed club cells". In the specimens investigated no intermediate stages between the true club cells and these "Drusen" cells are dis- cernible to support RASE 's ~iew. The name "Kornerdrlisen" or granulated cells used by RAUTHER (190?) is not an appropriate one as these cells do not always have granulated contents. As an abbreviation of the name given by STUDNICKA (1906) these cells may conveniently be called "Sac.;..cells", just as the term "club cells" has been adopted from "Kolben-formige Gebilde" of SCHULTZE, :w·. J" .s. ( 1861) • The present investigations show that out of the three genera of Callichthyidae under investigation these sac cells 132 are present only in Corydoras and are absent in Callichthys and Hoplosternum. In Corydoras they form a definite generic character and are present in all the species investigated. In Loricariidae they are present in young Plecostomus and Otocinclus and are absent in other genera studied. The sac-cells form an independent category of gland cells and are easily distinguishable from club cells in their struc- ture. Club cells have their nucleus invariably placed in the centre and are filled with homogenous or slightly granular contents which become vacuolated in the large cells, take acid stains and never open to the exterior. The sac4cells in Callichthyidae and Loricariidae have their nucleus towards the . peripheral wall at the base, their contents homogenous in early stages later become granular and always open to the exterior. The contents stain brilliant red with acid fuchsin, red with eosin and black with iron haematoxylin. Both the homo- genous and granular contents are not stained by thionin or toluidin blue. The ·sac-cells vary in shape. The typical fonn is like a bag or sac with a broad base which lies in the middle layer of the epidermis and with a long neck which opens to the exterior as in Corydoras. In young Pleoostomus these cells are round and occupy the entire thickness of the epidermis and have a very short indistinguishable neck which opens to 133 the exterior. In Otocinclus nigricauda BLGR. they are either oval or oblong in their early stages and occupy the entire thickness of the epidermis, and when advanced in their development they shrink in size and become bag-like with long necks and with granular contents confined to the middle of the cells in the rorm of a bag within the main cell. PAWLOWSKY (1911) regards the sac.cells as belonging to the category of "albumen glands" ("Cellulae albuminoidae acidophilae) containing albumen contents. NICKERSON (1893) who noticed their pres.ence in the epidermis of Lepidos teus regards their function as "secreting some components of slime other than mucus". In Callichthyidae RAU1l'IIBR (1907) suggests that the "granulated cells provide albumen containing secre- tions which may have poisonous properties". He regards the dimorphism of skin glands as "analegous to that in Amphibia" which posse.ss "both the secreting and slime glands". The wide distribution of sac cells in different orders of fishes does not attribute any specific character to the existence of such cells. They secrete an albuminous granular substance which forms a constituent of slime. Other elements of epidermis in Siluroidea such as fibro- blasts, pigment cells and cutaneous sensory organs are similar to those described by various investigators in other speices of fishes. The epidermis is separated from corium by a fine structureless membrane called basement membrane ("Basal mem- branu"). Such a membrane has been described by WRIGHT (1884) NUSBAUM u. KULCYZKI (1906), OXNER (1905), RAUTHER (1907), MERKEL (1909) and others in various species of fish. :MERKEL (1909) states that "these membranes are taken either as a product of connective tissue or as a cuticular excretion of epithelia and even double formation from both sources has been described for it". NUSBAUM (1907) ascribes an epidermal origin to it; HASE (1911) and others regard it as belonging to corium. HASE (1911) follows MERKE~'s (1909) suggestion "to drop the expression 'Basal Membrane" and ttto use for it the expression 'Grenzhaut or Membrana terminans' "• In Siluroidea the basement membrane belongs to the corium and is either structureless or may have a few connective tissue cells embedded in it and has been figured as such by RAUTHER (1907). But it seems superfluous to invent a special name and hence the old term basement membrane has been retained. The · oorium Eihows di.fferentiation into two layers, an outer and an inner, which are sharply defined in the regions where scutes are developed, the scute forming a boundary between the two layers. The outer layer forms a loose net- work of fibres, while the inner layer has bundles of connective tissue fibres, which form three sets according to the diree- tions in which they run. Two sets run in horizontal planes 135 parallel to the sur-face of the skin, while the third consist- ing of vertical bundles runs between the horizontal ones. A similar condition has been described by NICKERSON {1893) in Lepidosteus. The blood vessels and pigment cells lie under the basement membrane, and there are connective tissue cells in both the layers. (b) Dermal Skeleton. HERTWIG (1876) according to his "concrescence· theory" regards in Loricariidae and Callichthyidae, the "simplest form of skin ossification" a "small quadra.te bone which on one bone ring has one little tooth" as homologous with the conical apex and basal plate of a placoid scale. From this ground form. he derives all other parts of skin skeleton by "fusion of small plates" and "by extension of ossification to deeper layers of cutis". The development of fin platelets independent of denticles, which develop later, he regarded as a secondary condition "from which phylogenetic development cannot be deduced". KLAATSCH (1890) supports HERTWIG's views but considers that "each scale o:f' a fish represents a single denticle of which the basal plate has become enlarged". NICKERSON (1893) interprets the separate development of the denticles and the bony plate in Lepidosteus as secondary and compares the ganoid scale to the placoid denticle, and considers 136 the basal plate of the latter has given rise to the scale. I 'r'l't - ROSE (1897), KERR (1903) and DEGENER (1924) have supported HERTWIG's theory by their investigations. WALTHER (1883) and CARLSSON (1894) regard the tooth-bearing bones to be genetically of double origin: firstly, "cement bones formed by the fusion of the cement plates of the teeth", secondly, "connective tissue bones :formed in subcutaneous connective tissue without previous formation of teeth". CARTER (1919) considers that "the bone to which the teeth or denticles are attached is not independently developed, but is an extension of the denticle cone". According to WILLI.ATuBON (1849) the oosmoid scale arose by the fusion of large numbers of denticles and their com- bination with a bony plate developed below. In Megalichthys each pulp c~vity of the cosmine is supposed to indicate a single denticle. Macropoma he regarded as representing an intermediate stage in the formation of cosmine, as here the denticles are fused to the underlying bone and either stand up as sharp teeth on the scales or are more or less blunt protruberences on the cranial bones and form superficial ornamentation. WILLIAMSON (1851), therefore, while investigating the structure of the dermal denticles of Loricaria oataphracta L., remarked that "It only requires the tooth to be fixed instead of movable, and depressed instead of accuminate, in order to 137 render it the exact homologue .of one of the areolae1 in the kosmine of Megalichthys". Further he regards the large pointed teeth covering the scales of Macropoma mantelli to "closely resemble those of the Siluroid fish. They differ little in the two cases, beyond the fact, that whilst in the Siluroid the teeth have contracted bases and are movable in the Macropoma those bases are expanded and firmly fixed upon the upper surface of the scale". GOODRICH (1907) adheres in general to WILLIAMSON'S views as to the independent development in the dermis of a bony plate to which the overlying denticles ma.v have subsequently become fused and considers that "The comparison of the denticle and ganoid scale with the conical apex and basal plate of the placoid denticle seems to be fundamentally wrong. The basal plate is merely an extension of the denticle cone, there is no reason to think that it can every develop separately". These views of GOODRICH (1907) have been corroborated by SEWERTZOFF (1926, 1932) in the development of Acipenser and Polypterus and by MOY-THOMAS (1934), the latter author observes that "in development there is always a marked and easily recog- nised distinction between tooth and bone". (1) Areolae in Megalichthys "are the cup-shaped spaces with multitude of ramifying tubules" given off from the ascend- ing branch of the second layer of canals. The main branches of these tubules ascend and distribute their terminal rami- fications immediately under the ganoin; (WILLI.AI~SON, 1849). 138 HERTWIG (18?6) and WILLIAMSON (1861), thus, interpreted the structure of the dermal skeleton of armour clad Siluroidea in accordance to their own respective theories. Neither of them however studied its development. It is, therefore, necessary to discuss the merits of their interpretation in the light of the present investigations. The ventral scutes both in Loricariidae and Callichthyidae develop later than the body scutes and appear first in the posterior region in front of the anus and gradually spread over the whole of the ventral surface. The body scutes in both the families start appearing from the posterior end of the body. The scutes whether on the body or on the ventral skin develop independently of the denticles and appear as a thin sheet of calcareous substance secreted in the midst of the internal layer of the cerium. The connective tissue cells (scleroblasts) lie all round the margin of the developing scute and ultimately become enclosed within it as bone cells. The calca~eous matter is not deposited where the blood vessels enter the acute and the openings persist to form the Haversian canals. The scale increases in size and thickness by the calcification of the corium fibres. Vertical ascending and descending fibres of connective tissue run into the substano of the ventral scute. On the body the anterior end of the scute, as increase in lateral extent takes place, becomes deeply embedded in the corium and is overlapped by the posterior end of the preceding scute. 139 When the denticles appear their pediments become attached to the underlying scute. The process of calcification con- tinues and in between the pediments bony trabecuale are formed which join the bases of the pediments and form an upper layer. In Callichthyidae the cells of the outer layer of corium, after the attachment of: the denticles, secrete on the surface of the scute a homogenous, hyaline layer of cell-free osteoid tissue which encloses the pediments and the vertical fibres of the corium. This layer of osteoid or "cell free bony tissue" forms superficial ornamentation of the scute and is developed in- dependently of the denticles. It is traversed by vertical tubes which start from the upper border, frequently bifurcate into fine branches and end into the bony substance of the scute. AGASSIZ (1833-43) described this layer as "enamel". HERTWIG (1876) regards it as "modified bone substance with tooth bone tubes ("Zahnbeinrohrchen") as they have been found and described in the bones of several fish genera", and suggests that "as the surface of the scale is covered by a thin connective tissue layer which is rich in cells one would expect that from these cells processes would penetrate into the tubes". The present investigations, however, show that this layer is neither enamel nor ganoin as it is not dissolved by acids; and is not comparable to cosmine as it does not possess oosmine 140 tubes and the vertical tubes are not dentinal tubes or lepidosto- id tubes as described in other fishes as they do not contain prolongations of any cells but contain bundles of connective tissue fibres the course of which can be traced by different stains from the outer layer of corium into the substance of the scute. Growth of the scute takes place at the posterior tooth carrying border by the addition of fresh bony layers produced by calcification of the inner layer of corium and by develop- ment of fresh denticles and their attachment to the underlying plate of bone by their pediments. The largest denticles at the posterior border, therefore, are the youngest in age. The development of the fin plates takes place by a similar process to that of the body scutes and the denticles become secondarily attached to the underlying plates by their pedi- ments. On the ventral skin both in Loricariidae and Callichthyi- dae the denticle buds make their appearance earlier than the scutes, but before the denticle erupts the scute has appeared. The- denticle develops in the outer corium beneath the basement membrane, the basal layer of the epidermis forming the enamel organ, consisting of an inner layer of ameloblasts and an outer layer of enamel epithelium., and the mesoblastic cells arrange themselves to form odontoblasts. The dentine 141 appears in between the am.eloblasts and the odontoblasts. The enamel organ embraces the denticle papilla up to its base and is of uniform thickness and is not like the enamel organ of Hake and others (TOMES, 1923) where on account of partial investment of enamel the cells of the enamel organ opposite to the tip of the denticle attain large size while below that point they exist in a "sort of rudimentary condi- tion". The dentine matrix shows fibrillar structure which is clearly discernible both in ground and decaloified sections especially in Callichthyidae where the absence of the dentine tubes does not obscure the texture of the dentine. In 1891 von EBNER pointed out that a fibrillar structure can be detected in decalcified dentine. MUMMERY (1891) has demon- strated that "the connective tissue fibres from the pulp pass into .the forming dentine" and "form a meshwork or foundation on which calcification takes place". STUDNICKA (1906, b) demonstrated the fibrillated composition of the dentine of the developing placoid scale of the embryo of Spinax niger and noticed the communication of its connective tissue fibrils with those of the corium and re.marks that the "tooth with the exception of its enamel layer is almost nothing else than a connective tissue papilla calcified on its surface". He regards the penetrating fibrils "as analogous to Sharpey's fibrils in bony tissue". The investigations of von EBNER (1891), MUMMERY (1891, 1924), von KORFF (1905), STUDNICKA (1906, b) and others, therefore, leave no doubt that "in fish as well as in several orders of the mammalia and in man con- nective tissue forms a framework or basis to the dentine". (MUMi~RY, 19 24) • In the dermal denticles of Loricariidae and Callichthyidae the present investigations show that the connective ti"ssue forms an organic foundation in which calcification takes place and the fibrillae run parallel to the.long. axis of the denticle, terminate at· the amelo-dentinal junction and at the base con- tinue into the fibrils of the connecting area and· then merge into the substance of the pediment. The calcification of the dentine stops at the point up to . which the enamel organ had extended. Below this point the base of the denticle is prolonged into connective tissue fibres which come out of the dentinal papilla and attach the denticle to the underlying plate. As soon as the denticle is attached its tip pierce.a the epidermis, although its pediment is not yet formed. A somewhat similar condition has been described by NORMAN (1926) in the teeth of the larval eel, and by MOY-THOMAS (1934) in the teeth of larval Belone where the teeth pierce the epidermis before attachment to the underlying bone. The pediment, pedestal or bone of attachment develops in the connective tissue prolongation of the denticle base which assumes at first the transluscent appearance of tissue on the borderland of calcification, and then calcification starts from near the base of the denticle and proceeds towards the plate of bone. The area between the base of the den- ticle and the developing pediment remains uncalcified and forms a connecting area of fibrous tissue. The pediment is thus enclosed both internally and externally by the connec- tive tissue fibres of the connecting area, which, thus represents neither the "capsular expansion of the membrane which covers the scale" as interpreted by WILLI.AMSON ( 1851) nor "the connective tissue fibres of the boundary of the place of articulation" as suggested by HERTWIG (1876) but are part of the same dentine papilla on which develop the denticle and the pediment. In socketed denticles, as has also been pointed out by CARTER (1919), the base of the denticle is at first straight but gradually the inner surface of the connecting area be- comes converted into dentine and forms the basal prolongation into th~ cavity of the pediment. In anchylosed denticles (Pseudacanthicus) and the bristles (Xenocara) the connecting area calcifies when the denticle or the bristle is fully developed into a hard transluscent substance in which a faint outline of its fibrous texture is visible. TOMES (1923) regards the "bone of attachment" or the pediment as "more or less directly an outgrowth from the jaw 144 bone itself and which is, in some unseen manner, stimulated into activity by the proximity of the tooth", and according to the latest contribution to our knowledge by MOY-THOM.AS (1934) "Supporting pediments are shown to be only bony supports developed precociously in connection with teeth, and may be formed directly from the bone or separately as intermediate pediments and are in no way different from the remainder of the bone". CARTER {1919) definitely demon- strated the formation of the pediment and the denticle on one and the same papilla. According to the present investigations the connective tissue matrix for calcification of the different parts of the denticle explains their continuity. On one dentinal papilla the dentine appears first, the enamel next, pediment follows, then the basal prolongation of the denticle is formed to socket it and lastly in anchylosed denticles it is the connecting area that calcifies. The continuity of the denticle papilla is further explained by the fact that not only its pulp cavity but also the odontoblasts are con- tinued into the cavity of the developing pediment, (FIG. 31, ob). The calcification of the pediment starts between the denticle and the underlying bone and this may have led to the conception that it is of independent origin but the matrix, in which it develops, is laid before its calcification commences, on the same papilla as produces the denticle. 145 CARTER (1919) explains the difference in the structure of the different parts of the papilla as "due to some in- fluence exercised by the investing tissues". To his ex- planation it may be added that the area invested by epithel- ium is surrounded by amelbblasts externally and odontoblasts internally and during the process of calcification no con- nective tissue cells are enclosed, while the pediment during its calcification grows into the inner corium layer and is surrounded by abundance of connective tissue cells which become enclosed within and form the bone cells (FIG. 42, p. ), while the calcification of the connecting area in the outer cerium produces an osteoid tissue of homogenous hyaline character. The "marked and early recognised distinction between the tooth and bone" as has been assumed by MOY-THOMAS (1934), according to the present investigations, does not exist as the bony pediment develops on the same papilla as forms the denticle. Development of the dermal denticles, therefore, pro- ceeds on similar lines as that of the pladoid scale and con- sists of an enamel cap and dentine body with pulp cavity, corresponding to the enamel tip, the dentine body and the pulp cavity of the placoid scale, and its basal plate is represented by t he pediment as it is formed by the basal prolongation of the denticle. It is not the scute as be- -i lieved by HERTVvIG (1876) that is homologous to the basal 146 plate as there is no evidence available to show that it is formed by the extension of the pediment. BAUDELOT (1873) regards the scale of the type of Hyposto- ma to establish a kind of transitional stage between the scales ~ of Selachii and those pf the other bony fishes. HERTWIG (1876) c0nsiders the placoid scales and the skin teeth of Siluroidea remarks as homologous formations and ;that the "latter must be derived from the former." NICKERSON (1893) considers the denticles in A- e Siluroidea and Lepidosteus to be present inkdegerlfate condition. KGSCHKAROFF (1907) regards the denticles in Loricaria as simi- lar to placoid scales and considers them to be primary struc- tures as he adds that "it is impossible to suppose that once lost placoid scales appear again and the fusion process of their basal processes could begin again.'' GOODRICH (1909) gives three opinions: (1) p. 356: ''True denticles with dentine cone and pulp cavity do, however, occur in large numbers on the der- mal skeleton of Siluroids, where they are movably articulated to the underlying bones. This puzzling fact can at present only be accounted for on the supposition that the Siluroidei have been derived from ancestral Teleosts in which the .dermal denti- cles were still present, as they are in Polypterus and Lepidos- teus;" (2) p. 379: "The Siluroidei present some most striking superficial resemblance to the extinct Cephalespidae and Coecoa- teidae and so close is the likeness (especially among the Clariinae and Loricariidae) as even to deceive so acute an ob- server as HUXLEY (1861). It is now generally admitted that the resemblance between the Siluroidei and the Devonian fossils is 146 plate as there is no evidence available to show that it is formed by the extension of the pediment. BAUDELOT (1873) regards the scale of the type of Hyposto- ~ to establish a kind of transitional stage between the scales of Selachii and those pf the other bony fishes. HERTWIG (1876) considers the placoid scales and the skin teeth of Siluroidea remarks as homologous formations and ;that the "latter must be derived from the former. 11 NICKERSON (1893) considers the denticles in A. e Siluroidea and Lepidosteus to be present inkdegerlfate condition. KQSCHKAROFF (1907) regards the denticles in Loricaria as simi- lar to placoid scales and considers them to be primary struc- tures as he adds that "it is impossible to suppose that once lost placoid scales appear again and the fusion process of their basal processes could begin again. '1 GOODRICH (1909) gives three opinions: (1) p. 356: "True denticles with dentine cone and pulp cavity do, however, occur in large numbers on the der- mal skeleton of Siluroids, where they are movably articulated to the underlying bones. This puzzling fact can at present only be accounted for on the supposition that the Siluroidei have been derived from ancestral Teleosts in which the .dermal denti- cles were still present, as they are in Polypterus and Lepidos- teus;11 (2) p. 379: "The Siluroidei present some most striking superficial resemblance to the extinct Cephalespidae and Coecoa- teidae and so close is the likeness (especially among the Clariinae and Loricariidae) as even to deceive so acute an ob- server as HUXLEY (1861). It is now generally admitted that the resemblance between the Siluroidei and the Devonian fossils is due to convergence''; (3) footnote on p. 369: "It is of course possible that the denticles on the surface of the body of Siluroidei have been,so to speak,reacquired; the bony plates on which they rest seem to be secondary since they of- ten overlie the normal bones of the skull". It seems, as remarked by REGAN (1924) ''curmous that the skin of fishes, whose more remote ancestors had cycloid scales and whose immediate ancestors were naked, should have retained or regained the power of forming denticles of the Selachian type over the whole surface of the body". He regards the dermal dentia.les an exception to DOLLO' s ''law of irreversi- bility" (1893) which states that ''an organ, once lost, cannot be redeveloped in its original form and that an organ, once changed, cannot revert to its original structure", and consi- ders them to be''a genuine example of the redevelopment of an organ that has been lost, although, perhaps, the fact that the Loricariidae have teeth in the jaws makes the development of structures homologous with teeth on the bony plates cover- ing the . body somewhat less marve~ous'' • It is hard to say whether the dermal denticles of armour clad Siluroidea are placoid scales which have re-appeared or are simply new for-in mations similar tf6 structure to Selachian .teeth. Such forma- tions, though of rare occurrence, are found in other Teleostei such as the denticles on the snout of Xiphius gladius des- cribed by CARTER (1919). 148 The present investigations do not support KASCHKAROFF's view as no fusion process of the basal processes of the der- mal denticles takes place. In armour clad Siluroidea the independent development of the scute and subsequent attach- ment of the denticles seem to recapitulate the origin of the cosmoid scale as interpretted by WILLIAMSON (1849), but the denticles, though their pediments join to-gether by bony trabeeulae to form an upper layer of the scute in Loricariide do not contribute to the formation of the surface ornamenta- tion end ·:the ,- upper h.ylineJ ·· a.ay,'er ·~ that f'ornis the surface or- namentation on t1~e scutes in Callichthyidae is of independent origin and is not formed by the fusion of' denticles. The dermal denticles in armour clad Siluroidea, therefore, neither contribute to the development of the scute nor to the formation of the surface ornamentation and, as has been suggested by RAUTHER (1929), are ''only attribute and not a part of skin ossification''. SUMMARY Integument 1. Callichthyidae, though armoured with bony scutes and dermal denticles possess club cells in their epidermis, which in young speci.tfl.ens of Callichthys, Hoplosternum and Corydoras lie in a single layer, but which, in old specimens, occur in two to three layers as abundantly as in other Siluroidea, such as Doras, The fin spines of Callichthyidae, though thickly studded with dermal denticles, still possess club cells in their epidermis. The presence of club cells, which are a characteristic feature of Physostomi, form posi- tive evidence of the physostomian character of Callichthyidae, and shows that in skin structure they are not far removed from Doradidae. 2. Loricariidae do not possess club cells in their epidermis at any stage. The absence of these cells does not necessarily differentiate them from Physostomi as these cells are lacking in the family Salmonidae. To OXNER's statement that "within the Order Teleostei club cells occur only in Physostomi excepting the family Salmonidae in which the cells are lacking entirely" the family Loricariidae should be added as forming another exception. SUMMARY Integu.m.en t 1. Callichthyidae, though armoured with bony scutes and dermal denticles possess club cells in their epidermis, which in young speci.p].ens of Callichthys, Hoplosternum and Corydoras lie in a single layer, but which, in old specimens, occur in two to three layers as abundantly as in other Siluroidea, such as Doras, The fin spines of Callichthyidae, though thickly s~udded with dermal denticles, still possess club cells in their epidermis. The presence of club cells, I which are a characteristic feature of Physostomi, form posi- tive evidence of the physostomian character of Callichthyidae, and shows that in skin structure they are not far removed from Doradidae. 2. Loricariidae do not possess club cells in their epidermis· at any stage. The absence of these cells does not necessarily differentiate them from Physostomi as these cells are lacking in the family Salmonidae. To OXNER 's statement that "within the Order Teleostei club cells occur only in Physostomi excepting the family Salmonidae in which the cells are lacking entirely" the family Loricariidae should be added as forming another exception. 11 150 3. Argiinae, a sub-family of Loricariidae, though naked on their body, show the same structure in their epi- dermis as the other families of Lorioariidae and do not possess any club cells. It is further evidence to show that the nakedness of the body is a secondary character. 4. The presence of club cells in armoured Callichthyi- dae and their absence in naked Argiinae evidently shows that the correlation between the frequency of the club cells and the dermal skeleton as assumed by BAUTHER (1907) does not exist. 5. Sac-cells, a term proposed for the "Sackformige serose Drusen" of STUDNICKA (1906, , a), "Kornerdrusen" of RAUTEI:H;R (1907) and "Offene Kolben" of HASE (1911), are cells with albuminous contents which .may either be homogenous or finely or coarsely granular, with a peripheral nucleus lying at the base and stainable with acid stains. The cells always open to the exterior by a neck either short or long. They are round, oval or bag-like. In Callichthyidae these cells, in the three genera studied are found only in Cory- doras and appear to form. a definite generic character since they are present in all the species investigated. They are absent in Callichthys and Hoplosternum. In Loricariidae they are present in Otocinclus and in young specimens of Plecostomus. The sac_cells form an independent category of cells and are distinguishable from club cells which do not 15ill open to the exterior and in Siluroidea have their nucleus in the middle. Dermal Skeleton 6. Loricariidae (excepting Argiinae) and Callichthyi- dae are armoured on their body with rows of overlapping scutes. The ventral skin in Loricariidae is either naked or is provided with small scutes. .Among Callichthyidae, Callichthys and Hoplosternum have a naked ventral skin, while in the adult of all the species of Corydoras studied the ventral skin possesses either a few or a large number of irregular bony scutes with one or more denticles, thus forming a definite generic character. 7. Scutes have a true bony texture with bone cells and Haversian canals. In Loricariidae and Callichthyidae the bone cells are oval, round or quadrate with numerous fine ramifying canaliculi which intertwine with those of neighbour- ing cells. In Deras and~ besides the ordinary bone cells, there are, in the upper layer of the scute and in the nuchal shield and fin spines, bone cells which have their canaliculi vertical to the long axis or the cell; and there are in the lam.ellae of the outer layer, abundant radiating canaliculi, either with or without, a very much reduced body of bone cell. Thus one can trace the stages by which the typical bone cells 152 become reduced, by elimination of their body, to radiating canaliculi which according to QUEKETT (1846) are found in the "majority of the bones of fishes". 8. The scutes develop independently of denticles and appear in the inner layer of corium and increase in thickness and lateral extent by calcification of the connective tissue fibres of the corium. In the Callichthyidae the superficial layer, secreted by the cells of the outer layer of coriu.m, is homogenous and hyaline in texture. It is without bone cells and its striations,parallel to the surface of the acute, indicate zones of its growth. This layer is traversed by numerous vertical tubes, which start from the outer border, frequently bifurcate and branch and end i n the substance of the scute. These vertical tubes contain prolongations of the connective tissue fibres of the outer layer of corium but not from any cells . This layer is neither en amel nor ganoin as it is not dissolved by acids , and is not cosmina as it neit her ari ses by fu i .on of denticles no r possess es any sy stem of cosmine t ubes. I t is simply a layer of osteoid cell-free bone- t issue such as i s oft en .met wi th in Teleostei such as Plectognaths and others. 9 . The soutes of armour clad Siluroidea , therefore, differ from ganoid scale histol ogically , as there is neither ganoin nor cosmine , and develppmentally, as they arise in the 158 inner cerium. layer and not, as in the case of Lepidosteus (NICKERSON, 1893), in the o·uter. Whatever resemblance there is between the scutes of Loricariidae and Lepidosteus is due to a similar process of calcification of the corium which has the same structure in both cases. 10. The "lepidine tubules" of' WILLIAMSON (1849) are not tubules "for the general nutrition of the scale", but are fine uncalcified connective tissue fibres which both in Loricariidae and Callichthyidae are seen traversing the lamellae of isopedine. 11. Growth of the scute takes place at the thin posterior border of the scute which covers the thick anterior border of the succeeding acute; it is here that fresh den- ticles are formed so that the denticles at the posterior border are the last to be formed and are stronger and longer than those on the other part of the acute. 12. The denticles are true tooth-like str.uctures, with enamei cap, a body ~f dentine, tubular or non-tubular, en- closing a pulp cavity and a pediment. In Loricariidae the denticles have tubular dentine, and in all sub-families, excepting Argiinae, are socketed. They reach their fullest development in Pseudacanthicus and Xenooara where on sane parts of the body they are anchylosed to their pediments. In Argiinae the body is naked and the dentioles, which are confined to the fin spines and fin rays are not socketed and 154 have tubular dentine in their distal part and hard non- tubular dentine at their base. The bristles ori the operculum of Loricariidae have the same structure as the denticles. They are either movably articulated (Pseudacanthicus) or are firmly anchy- losed (Xenocara). The base of the bristle is socketed. Their bony pediments are implanted in bony plates which are united with one another by connective tissue fibres and are in continuation of the series of plates on the ventro- lateral border of the operculum. The plates anterior to bristles bear ordinary denticles. The bristles in Xenocara are not movable but the plates bearing them are movably connected with one another. 13. In Lorioariidae the denticles are absorbed and renewed both on the fins and the scutes. In Callichthyidae they are renewed on the fin spines and the ventral scutes but not on the lateral scutes. In young Callichthyidae the denticles on the lateral scute are present in as much abundance as in Loricariidae but in the adult, .as the denticles are not socketed, the;y fall off with rough handling, or are absorbed and not renewed. At the posterior border they form a continuous regular row. In well preserved specimens of Corydoras, both young and adult, the denticles are uniformly distributed over the surface of the scute. tubules. The dentine is hard and without any trace of 14. The denticle, its pediment and its connecting area develop as one unit on one and the same papilla of outer cerium with a matrix of connective tissue fibres. Dentine appears first, then enamel. The base of the denticle is then prolonged into calcifying connective tissue which attach- es %m the denticle to the underlying plate of bone. The denticle ruptures, and it is then that the pediment makes its appearance as a calcification in the connective tissue matrix formed by the prolongation of the base of the denticle. The area between the base of the denticle and the pediment remains uncalcified and forms the area of the connective tissue fibres or calcifies into hyaline cell-free bone tissue. The connective tissue matrix forming one papilla ex- plains the development, absorption and renewal of all the parts of the denticles as one unit. The base of the denticle is at first flat, and in the socketed denticles the connecting area on the inner side of the pediment in continuation of the denticle base calcifies to form the basal prolongation to socket the denticle. 15. The enamel is tubular and is not confined to the tip but extends over the dentine up to one half or one thir d of the length of the denticle. The pulp cavity extends into its substance in varying degree and gives off branches. The dentine shows a fibrillar structure, the fibrillae running parallel to the long axis of the body of the denti- cle. These fibrils merge into the fibres of the connective area and then pass into the substance of the pediment, which is bony and has bone cells. 16. The denticle with its pediment is homologous with the placoid scale, its pediment and not the scute, corres- ponds to the basal plate of the latter as it is formed by the extension of the base of the denticle. The difference in the structure of the denticle, its pediment and the connect- ing area is due to the investing tissues. 17. BEGAN (1924) regards the dermal denticles as an exception to DOLLO's "law of irreversibility" and considers them to be a "genuine example of the redevelopment of an organ that has been lost". It is hard to say whether the dermal denticles are placoid scales which have reappeared or simply new formations similar in structure to Selachian teeth. 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(1906)a Drusenzellen und Cuticulargebilde der Epidermis von Lepadogaster. Anat. Anz. 29 • . , (1906)b Uber Kollagene Bindegewebsfibrillen in der Grundsubstanz des Hyalinknorpels, im Dentin und im Knochengewebe. Anat. Anz. 29 • .. (1906)c Uber die Anwendung der Methode von BIEL- SCHOWSKY zur Impregnation von Bindegewebsfi- brillen besonders im Knochen, Dentin und Hyalin- knorpel. Zeitschr. Wiss . Mikr . 23 (1907)a Die radialen Fibrillensysteme bei der Dentinbildung und im entwickelten Dentin der Saugetierzahne •• Anat. Anz. 30. tt (1907)b Uber einige Grundsubstanzgewebe. Anat. Anz. 31. (1909)a M Zur Losung der Dentinfrage. ·Anat • Anz • 34. (1909)b. Vergleichende Untersuchengen uber die Epi- dermis der Vertebraten. Anat. Hefte. 39. ·, - ~' TIMS, H.W.M. (1906) The Development, Structure and Morphology of the Scales in some Teleostean Fish. Quart. Journ Micros . Sci. N.S. 49. T0rl1ES, c.s. (1874) studies upon ··trre ·A'ttacbme.nt of Teeth. Trans. Odont. Soc. 7. ( 1898) 1~11_ tD~ ~ti.rlbqt11J3..~ ::,'~~S\· D~Y..:~loB_~~1l~j .of the Enamel of Elasmobranch Fishes. Phil. Trans. Roy. Soc. Lona. (B) 190. (1923) Manual of Dental Anatomy. VAILL.ANT, L. (1895-96) Essai Monographique sur les Silures der Genre Synodontis. Nouv. Arch. Mus. Hist. Nat. Paris, (3) 7 & 8 . VAILLANT, L. & PETTIT, A. (1898) Sur la Structure du tegument chez le Synodontis schalli BLOCH. Bull. Mus . Nat. Paris, 4. WALTHER, J. (1883) Die Entwickelung der Decknochen am Kopfskelet des Hechtes. Jen. Zeitschr. Naturw. 16. WILLIAMSON, w.c. (1849) On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish. Phil. Trans. Roy. Soc. Lona. Ft. II. ------~(1851) Investigations into the Structure and Deve- lopment of the scales and bones of Fishes. Phil. Trans. Roy. Soc. Lond. Ft. II. WRIGHT, R.R. (1884) On the Skin and Cutaneous Sense Organs of Amiurus. Proc. Cand. Inst. N.S. ab = ad = a:f = at = Q = 1 EXPLANATION OF PLATES. Lettering. ameloblasts; ame lo-dentinal junction; anal :fin; adipose tissue; base o:f the denticle; be, be' = bone cells; bee • cenaliculi o:f bone cells; !2!!! : basement membrane; br = bristles; Q.£2. = bristle plate; bs = body scute; bsa = anterior part o:f body scute; bsp = posterior part o:f body scute; bsr = regenerated part o:f body scute; bv = blood vessel; £. = cori um;. ea = , connecting area; 1. The sketches have been drawn under ABBE's camera lucida. The original plates have been retained :for publication and photographic cppies are submitted. cb = = = - - central area of bone in tubercle; club cells; calcified connecting area; central cavity of fin spine; cf, cf' = connective tissue fibres; eh = ci = cic - £!1 = £.Q. = £.£ = er = - - horizontal bundles of corium; inner layer of cerium; cavities in inner layer of corium; radiating canaliculi; outer layer of corium; papilla of _cerium; annuli on the enamel; ctc, ctc' : connective tissue cells; = d d' _, - = = = = = - - = vertical bundles of corium; denticle; denticles on carinate portion of the scute; basal prolongation of the denticle for socketing; space produced by absorption of denticle; dentine; dorsal; dentine tubes; enamel; ~ = fill = ~c • fil!Q. = ~ = epb = fil2.£. - L = fb = fc = 02., t£' fpa • !Pl2. = gg_ = fil: = he, h£.' h£Q = ml - - is = isl = 1 = lla = 1 '/ -'-If....• 174 enamel tubes; decalcified enamel space; enamel enamel organ; epidermis; basal layer of epidermis; ' ordinary cells of epidermis;' fibrilla.e of dentine; fibroblasts; connective tissue fibres joining the fin plates; = fin plates; distal part of fin plate; proximal part of fin plate; granulated or sac-cells ; contents of sac-cells; = Haversian canals; opening of Haversian canal; osteoid layer of body scute of Callichthyidae; isopedine; lamellae of isopedine; lamellae of bone; anterior opening of lateral line canal; llc = = - • • n = nb = = = nh - - • 0 • . - :r or = = 12,' 12, I ~ ._. ' , ·~ t ~' pc~ pea = pcb = - lateral line canal; posterior opening of lateral line canal; connective tissue fibres forming the so called "lepidine" tubes of WILLIAMSON . mucus cells; middle layer of scute; nerve bundles; end - bud; nucleus; nerve ganglion; nerve - hillock; Nasmyth membrane; 6pening of nerve-hillock; odontoblasts; surface ornamentation of the scute; odontogenic zone; peidment; prolongation of dentinal papilla for attachment to underlying plate of bone; = pigment cells; pulp cavity; branches of pulp cavity in enamel; = = = = - = = = = - - = = = Y. = = = = = = = = developing .pediment; pediment without denticle; pectoral fin; pectoral fin spine; cavity of pediment; pigment spots; pelvic fin; supporting cells of cutaneous sensory organ; spine of body scute; serrations on the fin spine; sensory cells of cutaneous sensory organ; bony trabeculae; upper layer of the scute; ventral side; vascular layer of the scute; vacuoles; ventral ridge; articular facet for scute of the other row; ventral scute; anterior part of ventral scute; posterior part of ventral scute; vertical tubes. \ FIG. 1. FIG. 2. FIG. 3. FIG • . 4~ FIG. 5. FIG. 6. FIG. 7. FIG. 8. FIG. 9. FIG. 10. --- - · ,.._ 177 PLATE · I. Plecostomus sp.· 18 mm. . X 1400. L.S. Ventral skin. Epidermis. Plecostomus sp. 18 mm. X 1400. L.S. Ventral skin. Sac - cell. Plecostomus sp. 18 mm. X 1400. L.S. Ventral skin. Nerve - hillock. Plecostomus sp. 18 mm. x 380. One row of denticles on body scute. Plecostomus plecostomus L. 150 mm. x 680. L.S. Ventral skin. Epidermis and cerium. P. plecostomus L. 150 mm. x 680. L.S. Ventral skin. End-bud and Nerve-hillock. P. plecostomus L. 150 mm. x 380. Scutes on the ventral skin cleared in 10 % K.O.H. PLATE II. Plecostomus commersonii c.& v. 146 mm. X 2. Distribution of ventral scutes. P. commersonii C.& V. 146 mm. X 5. Body scutes. P. commersonii c.& v. 146 mm. X 12. One body scute. FIG. 11. Plecostomus plecostomus L. 150 mm. X 80. L~$. Ventro-lateral skin showing ventral scutes lying over the anterior part of the lateral scutes. PLATE III. FIG. 12. P. plecostomus L. 150 mm. x 50. L.S. Ventre- lateral skin. Body scutes joined to-gether by connective tissue fibres. FIG. 13. P. plecostomus L. 150 mm. x 680. Bone cells from body scute. FIG. 14. P. plecostomus L. 150 mm. scute (gronnd). FIG. 15. P. plecostomus L. 150 mm. X 70. T.s. Body x 380. T.S. Body scute (ground), dermal denticle and Haversian canal. FIG. 16. P. plecostomus L. 150 mm. x 30. Upper sur- face of body scute treated with 10 %I K.O.H. showing denticles and sys- tem of Haversian canals. FIG. 17. P. plecostomus L. 150 mm. x 380. Stages in the development of denticles on ventral scute, (whole mount): 179 J., (a) early stage, . (b) advanced stage showing prolon- gation of the base of dentine cone to join the scute, (c) pediment is formed but the denticle is not yet socketed. PLATE DI. FIG. 18. Plecostomus plecostomus L. 150 mm. x 110. T.S. body scute before grinding showing the denticles, pediments and the connecting area. FIG. 19. P. plecostomus L. 150 mm. x 380. Tip of the denticle from ventral scute soon after the action of 2 % H.Cl~ FIG. 20. P. plecostomus L. 150 mm. x 380. Tip of the denticle from ventral scute one hour after treatment with 2 % H.Cl. FIG. 21. P. plecostomus L. 150 mm. x 380. Tip of the denticle from ventral scute twenty four hours after treatment with 2 % H.Cl. FIG. 22. P. plecostomus L. 150 mm. x 280. Denticle from body scute after treatment with 2 % H.Cl. FIG. 23. Plecostomus sp. 18 mm. x 1400. L.S~ body skin showing crowding of connective tissue cells to form dentinal pa- pilla. PLATE V~ . FIG. 24. Plecostomus plecostomus L. 65 mm. X 680. L.S. ventral skin showing developing denticle. FIG. 25. P. ulecostomus L. 150 mm. x 680. L.S. ventral skin from near base of pelvic fin. Development of the denticle. En- amel is just appearing. FIG. 26. P. plecostomus L. 150 mm. x 380. L.S. ventral skin from near pelvic fin. Develop- ment of the denticle. Enamel fully developed with prolongation of the pulp cavity. FIGS. 27 and 28. P. plecostomus L. 150 mm. x 380. L.s. ventral skin from near pelvic fin showing stages in the development of the pediment. FIG. 29. P. plecostomus L. 150 mm. x 380. FIG. 30. FIG. 31. P. 181 ventral skin from near pelvic fin. Fully developed denticle on the ventral scute. PLATE VI. plecostomus L. 150 mm. X 380. L.S. ventral skin from near pelt.ric fin. Ventral scute with two denticles, one just ruptured but without pediment, and wi-1/C' the ot~er~fully developed pediment but the denticle not yet socketed. Plecostomus sp. 18 mm. X 680. L.S. body skin. Ruptured denticle attached to body scute by connective tissue pro - longation of its base;o~ediment not yet developed. FIG. 32. P. plecostomus L. 150 mm. x 300. L.S. body skin. Developing denticle at the posterior end of the body scute. FIG. 33. P. commersonii C.& V. 146 mm. x 20. Surface view of pectoral fin spine. PLATE ·vII. FIG. 34. P. plecostomus L. 150 mm. x 90. T.s. pectoral 18:2 fin spine (ground). FIG. 35. P. plecostomus L. 150 mm. X 90. T.S. dorsal fin spine (ground). FIG .. 36. P. plecostomus L. 150.mm. X 180. L.S. un- divided ray of pelvic fin near its free extremity 1 ( J.e~J.C:., {',i. d J. FIG. 37. P. plecostomus L. 150 mm. X 180. L.S. un- divided ray of pelvic fin below its free extremity (decalcified). ! 1 PLATE. VIII. FIG. I 38. Ancistrus multiradiatus RANCK. 12 mm. X 1050. T.S. body skin. Nerve-hillock. FIG. 39. A. multiradiatus RANCK. 21 mm. X 1050. L.S. body skin. Nerve-hillock. FIG. 40. A. multiradiatus RANCK. 21 mm. X 380. L.S. body skin. Body scute with denti- cles (decalcified'). FIG. 41. A. multiradiatus RANCK. 21 mm. X 380. L.S. body skin. Part of body scute on which denticles have not yet de- veloped. FIG. 42. A. multiradiatus RANCK. 21 rm:n. X 680. L.S. body skin. Two pediments showing ' 183 the enclosing of connective -tissue cells (scleroblasts) as bone cells. FIG. 43. A. multiradiatus RANCK. 21 mm. X 1400. L.S. ventral skin. Epidermis and cerium. FIG. 44. A. multiradiatus RANCK. 21 mm. X 90. Body skin showing rows of denticles on the scutes. The posterior denticle is developing in four rows. PLATE IX. FIG. 45. Lipopterichthys carrioni NORM. 61 mm. X 680. L.S. ventral skin. Epidermis and corium. FIG. 46~ L. carrioni NORM. 61 mm. x 1400. L.S. ven- tral skin. Mucus cells. FIG. 47. L. carrioni NORM. 61 mm. x 180. Denticle ~ from dorsal surface of the pec- toral spine (whole mount). FIG. 48. L. carrioni NORM. 61 mm. x 180. Denticle from ventral surface of the pectoral spine (whole mount). FIG. 49. L. carrioni NORM. 61 mm. x 180. Developing denticle from ventral surface of 1 I I I pectoral spine (whole mount). FIG. 50. Xenocara hoplogenys GTHR. 90 mm. x 1400. L.S. ventral skin. Epidermis. FIG. 51. X. hoplogenys GTHR. 90 mm. x 80. Denticle from pectoral spine (ground). Fig. 52. ~. hoplogenys G'rlffi. 90 nnn. x 380. Tip of the denticle from pectoral spine (ground). FIG. 53. X. hoplogenys GTHR. 90 mm. x 40. Bristle from near operculum anchylosed to its pediment (ground). PLATE X. FIG. 54. X. hoplogeny~ GTHR. 90 mm. x 180. Bristle showing calcified connecting area (ground). FIG. 55. Pseudacanthicus serratus C.& v. 300 mm. x 30. T.S. body scute from caudal ped- uncle (ground). FIG. 56. P. serratus C.& V. 300 mm. x 80. Denticle on body scute showing calcified connecting area (ground). FIG. 57. P. serratus C.& V. 300 mm. x 120. Tip of the denticle (grouna). Length of the 18'6 aenticle. 5.3 mm., tip= 0.96 mm. FIG. 58. P. serratus C.& V. 300 mm. x 300. Tip of the denticle after action of 2 % H.Cl. PLATE XI. FIG. 59. P. serratus C.& v. 300 mm. x 6. Scutes from anterior part of the body. FIG. 60. P. serratus c.& V. 300 mm. x 380. T.S. body scute, showing fibrillar struc- ture of ~entine (decalcified). FIG. 61. P. serratus C.& v. 300 mm. x 180. T.S. body scute, showing denticle (de - calcified). FIG •. 62. P. serratus C.& V. 300 mm. x 8. Tip of the pectoral spine, outer border. (From skeleton in British Museum). FIG. 63. P. serratus C.& V. 300 mm. x 15. Portion of the plate bearing bristles near operculum. (From skeleton in British Museum). FIG. 64. P. serratus c.& v. 300 mm. x 20. L.S. Bristle (ground). (From skeleton in British Museum). 18.6 PLATE XII. FIG. 65. Otocinclus nigricauda BLGR. 21 mm. X 680. L.S. body skin. Epidermis and cerium with part of the body scute. FIG. 66. o. nigricauda BLGR. 21 mm. x 980. T.S. ven- tral skin. Epidermis. FIG. 67. o. nigricauda BLGR. 21 mm, X 1400. T.S. ventral skin. Epidermis showing mucus cells and sac-cells. FIG. 68. o. nigricauda BLGR. ~l mm. x 1400. L.S. body skin. Sac-cell. FIG. 69. O. nigricauda BI.GR. ~~ Jmn, x 680. Denticle. FIG. 70. O. vitta.tus RGN. 19 mm. x 680. L.S. body skin. Epidermis and corium. FIG. 71. O. vittatus RGN. 19 IJIID• x 300. L.S. body skin. Body scutes with denticles. FIG. 72. O. vittatus RGN. 29 mm. x 380. T.S. body scute showing denticle (ground). FIG. 73. O. vittatus RGN. 29 mm. x 380. Denticle from pectoral spine (whole mount). FIG. 74. o. vittatus Rgn. 29 mm. x 380. Denticle from pectoral spine after treated with 2 % H.Cl. (whole mount). 18!2 PLATE XIII. FIG. 75. Loricaria cataphracta L. 240 mm. x 680. L.S. ventral skin from near mouth. Epidermis and end-bud. FIG. 76. L. cataphracta L. 240 mm. x 30. Ventral skin showing arrangement of ventral scutes. FIG. 77. L. cataphracta L. 240 mm. x 150. L.S. ven- tral skin from near pelvic fin showing ventral scute (decalcified). FIGS. 78 and 79. L. cataphracta L. 240 mm. x 380. L.S. ventral skin from near pelvic fin showing ventral scute with one denticle (decalcified). FIG. 80. L. cataphracta L. 240 mm. x 90. Denticles on carinate part of the scute (ground). PLATE XN. FIG. 81. Arges brachycephalus GTHR. 50 mm. x 380. L.S. body skin. Epidermis and cerium. FIG. 82. A. brachycephalus GTHR. 50 mm. X 680. L.S. body skin. Epidermis. FIG. 83. A. brachycephalus GTHR. 50 mm. X 1400. L.S. body skin. Mucus cells. FIG. 84. A. FIG. 85. A. FIG. 86. A. FIG. 87. A. FIG. 88. A. FIG. 89. A. FIG. 90. A. brachycephalus GTHR. 50 mm. X 1400. L.S. body skin. Pigment cells. brachycephalus GTHR. 50 mm. X 1400. L.S. body skin. Nerve-hillock. brachycephalus GTHR. 50 mm. X 680. L.S. body skin. End-bud. guentheri BLGR. 90 mm. X 160. L.S. un- divided ray of pelvic fin (decalcifiefl). guentheri BLGR. 90 mm. X 160. Developing denticle from undivided ray of pelvic fin (whole mount). guentheri BLGR. 90 mm. X 160. Developing denticle from undivided ray of pee- toral fin (whole mount). PLATE x:v. ~entheri BLGR. 90 mm. X 160. T.S. un- divided ray of pectoral fin (decalci- fied). Denticles restored. FIG. 91. A. brachycephalus GTHR. 50 mm. x 160. L.S. un- divided ray of pectoral fin near its extremity (decalcified). FIG. 92. A. brachycephalus GTHR. 50 mm. x 160. L.s. un- divided ray of pectoral fin, middle 189 part (ground). FIG. 93. A. brachycephalus GTHR. 50 mm. x 380. L.S. un- divided ray of pectoral fin, showing denticle (ground). PLATE XVI. BLGR. FIG. 94. Callichthys pectoralis / 74 mm. x 680. T.s. body skin. Epidermis and corium. FIG. 95. Callichthys callichthys L. 140 mm. x 680 . T.s. body skin. End-bud. FIG. 96. c. callichthys L. 140 mm. x 6. Body scute, ventral surface. FIG. 97. C. callichthys L. 140 mm. x 6. Body scutes, upper surface. FIG. 98. C. callichthys L. 140 mm. x 80. Posterior bor- der of the body scute with denticles and Haversian canals seen from upper surface, a~er treatment with 10 % K.O.H. FIG. 99. q. callichthys L. 140 mm. x 70. T.s. body scute of upper row near its dorsal border (decalcified). FIG. 100. C. callichth~ L. 140 mm. x 380. T.S. body scute with epidermis, showing connec- FIG. 101. c. FIG. 102. c. FIG. 103. c. 19G tive tissue fibres from outer coriurn layer penetrating into the vertical tubes of the upper layer. PLATE XVII. callichth;r.::s L. 140 mm. x 380. T.S. body scute, upper layer with vertical tubes (ground). callichth;r.::s L. 140 rnm. X 70. T.S. body scute from its middle (ground). callichth;zs L. 140 mm. X 90. T.S. body scute, covered dorsal end (grotmd). FIG. 104. C. callichthys L. 140 mm. x 680. Bone cells from L.S. pectoral spine (ground). FIG. 105. C. callichthys L. 140 mm. x 680. Bone cells from T.S. scute (ground). PLATE XVIII. FIG. 106. c. callichth;r.::s L. 140 mm. x 300. Developing denticle from pectoral spine (whole mount). FIG. 107. c. callichth;r.::s L. 140 mm. x 300. Denticle from pectoral spine (whole mount). FIG. 108. c. callichthys L. 140 rrnn. x 300. Denticle from FIG. 116. H. littorale RANCK. 95 mm. x 680. L.S. ventral skin. End-bud. FIG. 117. H. littorale RANCK . 95 mm. x 10. Body scute showing regenerated part of one of tpe scutes of the upper row. FIG. 118. H. littorale RANCK. 95 mm. x 90. Upper view of body scute, posterior border with denticles and Haversian canals. FIG. 119. R. littorale RANCK. 95 mm. x 300. A portion of T.S. of body scute showing upper layer and Raversian canals (ground). PLATE XX. FIG. 120. R. littorale RANCK. 95 mm. x 30. Ventral end of the lower row of scutes, showing distribution of denticles. FIG. 121. R. littorale RANCK. 95 mm. x 380. A portion of T.S. body scute showing attachment of denticles (ground). FIG. 122. R. littorale RANCK. 95 mm. x 70. T.S. from middle of body scute (ground). FIG. 123. H. littorale RANCK. 95 mm. x 380. L.S. of · C "tcr 1 ~ e: pectoral spine showing serrations, (ground). 191 pectoral spine, after five minutes treatment with 2 % H.Cl. FIG. 109. c. callichthys L. 140 mm. x 300. Denticle from pectoral spine after treated with 2 % H.Cl. FIG. 110. q. callichthys L. 140 mm. x 400. T.S. (ground) of denticle dtrom pectoral spine show- ing fibrillar structure of dentine. FIG. 111. c. callichthys L. 140 mm. x 400. L.S. (ground) of denticle from pectoral spine, fi- brillae of dentine are continued into the fibres of the connecting area. FIG. 112. c. callichthys L. 140 mm. x 80. T.S. pectoral spine (ground). FIG. 113. c. callichthys L. 140 mm. x 50. L.S. pectoral spine from near its free extremity showing serrati9ns on its inner border and denticles on its outer (ground). PLATE XIX. FIG. 114. Hoplosternum littorale RANCK. 95 mm. x 380. L.S. ventral skin. Epidermis and cerium. FIG. 115. H. littorale HANCK. 95 rmn. x 680. L.S. ven- tral skin. Epidermis. I I 192. FIG. 116. H. littorale RANCK. 95 mm. x 680. L.S. ventral skin. End-bud. FIG. 117. H. littorale HAN.eK. 95 mm. x 10. Body scute showing regenerated part of one of tpe scutes of the upper row. FIG. 118. H. littorale RANCK. 95 mm. x 90. Upper view of body scute, posterior border with denticles and Raversian canals. FIG. 119. H. littorale RANCK. 95 mm. x 300. A portion of T.S. of body scute showing upper layer and Raversian canals (ground). PLATE XX. FIG. 120. R. littorale RANCK. 95 mm. x 30. Ventral end of the lower row of scutes, showing distribution of denticles. FIG. 121. R. littorale RANCK. 95 mm. x 380. A portion of T.S. body scute showing attachment of denticles (ground). FIG. 122. H. littorale HANCK. 95 mm. x 70. T.S. from middle of body scute (ground). FIG. 123. H. littorale RANCK. 95 mm. x 380. L.S. of \.,..;lrr l -~~-.e pectoral spine showing serrations, (ground). PLATE XXI. FIG. 124. Corydoras aeneus GILL. 50 mm. x 680. L.S. ventral skin. Epi'dermis and corium. FIG. 125. Corydoras elegans STEIND. 35 mm. x 680. L.S. ventral skin. Epidermis and corium. FIG. 126. Corydoras aeneus GILL. 50 mm. x 240. Ventral skin showing distribution of ventral scutes. FIG. 127. Corydoras agassizii STEIND. 45 mm. x 160. Ventral skin showing distribution of ventral scutes. FIG. 128. Corydoras krone~ RIBEIRS. 62 mm. x 160. Ventral skin showing distribution of ventral scutes. PLATE XXII. FIG. 129. Corydoras kronei RIBEIRS. 62 mm. x 380. Ven- tral scute with branching denticle. FIG. 130. Corydoras elegans STEIND. 35 mm. x 300. Ventral skin showing distribution of ventral scutes. FIG. 131. Corydoras paleatus JENY. 42 mm. x 80. Ventral skin showing distribution of ventral scutes. FIG. 132. Corydoras trilineatus COPE. 35 mm. x 160. Ventral skin showing distribution of ventral scutes. FIG. 133. Corydoras aeneus GILL. 50 mm. x 680. L.S. ven- tral skin showing connective tissue fibres running into the substance of the ventral scute. PLATE XXIII. FIG. 134. Corydoras aeneus GILL. 56 mm. x 680. Sarne as FIG. 133, another view of the section. FIG. 135. Corydoras aeneus GILL. 47 mm. x 17. Body scutes from anterior part of the animal showing distribution of denticles. FIG. 136. Corydoras aeneus GILL. 47 mm. x 60. Part of body scute from anterior part of the animal showing distribution of denticles. FIG. 137. Corydoras aeneus GILL. 22 mm. x 90. Body scutes from anterior part of the animal showing distribution of~ denticles . PLATE XXIV. FIG. 138. Corydoras aeneus GILL. 50 mm. x 90. T.S. body scute (ground). FIG. 139. Corydoras aeneus GILL. 50 mm. x 90. L.S. of body skin showing arrangement of the scutes (decalcified). FIG. 140. Corydoras aeneus GILL. 22 mm. x 680. L.S. of body skin (decalcified). FIG. 141. Corydoras aeneus GILL. 50 mm. x 380. L.S. ventral skin (decalcified) showing ventral scute with developing denticle. FIG. 142. Corydoras aeneus GILL. 50 mm. x 680. L.S. ventral skin (decalcified) showing ventral scute with a developing den- ticle which has just ruptured. FIG. 143. Corydoras aeneus GILL. 47 mm. x 680. L.S. body skin (decalcified). Part of body scute with denticles and upper layer. PLATE XX!.J. FIG. 144. Doras dorsalis c.& v. x 380. L.S. ventral skin. Epidermis and corium. FIG. 145. Doras dorsalis c.~ v. x 680. L.S. ventral skin. Nerve-hillock. FIG. 146. Doras weddellii CASTEIN. 118 mm. x 22. Body scutes. FIG. 147. Doras weddellii CASTEIN. 118 mm. x 40. T.s. 196 body scute (ground). FIG. 148. Doras weddellii CASTELN. 118 mm. x 40. L.s. body scute through the spine (ground). FIG. 149. Doras weddellii CASTEIN. 118 mm. x 380. T.s. (ground) a portion of body scute. FIG. 150. Doras weddellii CASTELN. 118 mm. x 680. Bone cells with vertical canaliculi. PLATE XXVI. FIG. 151. Rita rita H.B . 235 mm. x 380. L.S. ventro- lateral skin. Epidermis and corium. FIG. 152. Rita rita H.B . 235 mm. x 160. L.S. skin from head. Epidermis and corium. FIG. 153. Rita rita H.B. 235 mm. x 680. L.S. skin from head. End-bud. FIG. 154. Rita rita H.B. 235 mm. x 380. (a), (b), (c), club cells from L.S. skin from head. PLATE XXVII. FIG. 155. Rita rita H.B. 235 mm. x 90. T.S. nuchal shield (ground). FIG. 156. Rita~ H.B. 235 mm. x 380. T.S. (ground) of tubercle from nuchal shield. FIG. 157. Rita rita H.B. 235 mm. x 70. L.S. (ground) of ~ a serration from pectoral spine. I I I PLATE _ .. .... ~ -~ ...._,,r.l'~ FIG.2 FI G. 6 -~ . epb fb bv Pc VS hco SC PfS - - FIG. 8 II PLATE Il Pf FIG.10 dd bsp FIG 16 III PLATE Ill IS FIG.12 ~.de , I ....--PCd I I I ,-- pa --~c':!!.b)_ F I G. I 7 (C) U'I d dd FIG 16 bsa PLATE Ill bsp IS FIG, 12 .,., .C'l ! '·~·. \ ~ ' ' I • ' ' , I I • I' ' : 1 , ,, . ,:, ''· VII PLATE V n J ...... ('\ a.. a.. I"') 0 flG. 41 bSp C l ctc tT,C 'D fb u a... a... epb l/1 ..0 0 -"-1" -=- - -- - - -- --C) - - -L_,_ FI G.43 ,,l dd dd I / bsp d ~J I ,, I I/· "? \ ~1! IJ ,;/ .1 { tL \ ' J:- FIG.44 bsa FIG. -46 e FIG 52 FIG 5 1 X 0- K ~ 0- -u iii. \I' r a. l:a, :::r ("\ • ~ u fT1 >< 6 ,· e , e @ 8 "' '! / ,! ---bs pp FIG 62 FIG Sj me bs -- --<:~y- \ FIG. 67bv FIG. 68 ·bsa PLATE XII me = gc fb -=--~.- . . ..c-~ _-__.: ~ ---~ __ _..;... C FI G. 66 nc pea pp FIG. 79 ep co Cd p H "' -~-:-~~ ci H - - ~- - ·,. ·, 'lff;; . ' - ·= H ~ /~=----i= '' ~ ;, •' ~ - pp · ---- - ... -- -- .... - •• C7 Ill .... - - or.:. ~ - - ' ' ~·,,, c ... ~~~= - -· - _,,.. vs pea FIG. 79 C FIG.83 p pp he F G. 87 FIG. 82 pc nh SS FIG.86 FIG.84 d Cd FI G. 88 me n pc epb e pcb de dt pea pa Cd p FIG. 89 PLATE XV e p he f p co c i Cd CCV FIG. 90 fP ci - FIG. 91 fC ls pcb dt de pea p • -r -.,,, • .._ ,r . - ~- .. - - .... # .., """":.t .. - 'f" - --· bSd ng E'pb bsp FI G. 95 FIG. 96 FI G. 100 he FIG. 99 bm d co d bsd I iS bsP I I I I XVII XVIII PLATE FIG. 110 de de FIG. 107 nm pp he FI G. 109 F!G !14 SS cc ctc bv C FIG. 116 .. >>> bsp bsr y ~ .. .. f d :, ~ t bSd C t I t ' FI G. 117 L .L----- d FIG 115 ~ vt he FIG. 119 is XX i:, sr be VI he I") "O "O " C7' C, I") n _ j5 VI Q, C7' ,:, d FIG. t23 CCV hy he ---me [:·:i>~~;;)/:i~:\:-'.:,;1i::;;r.:/_·::;t.,>··r::·:· ·· .. · , " epc]. :.- '·. .<\' ·.:····. \'l::JJ~--, . . :· . ~· ..... •. .· •.. . · ... ':, .. ·. , .. .. . , . -~· ~ - .. · ... FIG.127 .FIG. .128 XXII '\. FIG 132 vs vs . CV vs FIG 133 cc I FIG 131 eh ci XXIII co p LA TE XXIII ......., r' bsp FI G. 137 Cd p ..a· -~ ,'-.. : .... \- ..... ............ - .. ~ ....... ;.. :.y .... - . :· ~ ._ - i -4 ~ s ! - ~-- .. f s z cc ob dd XXIV hY lp'd he FIG. 138 hy b~p ep d FIG. 139 dd vt p bs ctc cd FI G. 140 cc · c.tc gc d co cc b!> be PLATE XXV epb ''1'1 '111/IJ1ih11Ji) ' ''I!"! ""' , .., , ,11, .• ~11'/'11i1; ·',,,, 1 •. ;-,,1,,,. ;r - • I , .. ,, ' .,,.,, . :.I'' 11,',<, ,•, , ,,,.,.. ' I - I 'IU j !1f ,, ,, ,,,,,,.,, ,,., ,,,, ',, I I Ji, ·' I f :"" ' ' - I • 1 - r.' r ' . ·- ' ,11111\l llil\'illiTl'il'l'll':l/~/