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On the Dermal Pin-Rays of Fishes—Living and Extinct

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On the Dermal Pin-Rays of Fishes—Living and Extinct TH13 DEKMAL FIN-RATS OF PISHES. 465 On the Dermal Pin-rays of Fishes—Living and Extinct. By Edwin S. Oootlricli, ill.A., Pelluw of Merton College, Oxford. With Plates 35—41. CONTENTS. PAGE INTRODUCTION ...... 166 THE DERMAL RAYS or LIVING FISH .... 4GS Elasmobranchii. ..... 468 Holocephali . .471 Teleostei . .471 Amia . 477 Lepidosteus ...... '477 Polypterus . • . .478 Acipenser ...... 479 Dipnoi . 4S0 THE STRUCTUEE AND HOMOLOGIES OF THESE DERMAL RAYS . 483 THE DEKMAL EATS or FOSSIL FISH .... 492 Elasmobranchii ..... 492 Acanthodii . .492 Ichthyotomi ..... 493 Ostracodermi ..... 493 Arthrodira (Coccosteus) .... 494 Actinopterygii ...... 494 Isospondyli ..... 494 Protospondyli ..... 495 iBtheospondyli . 495 Cliondrostei ..... 495 VOL. 47, PA14T 4.—NKW SERIES. GG 466 EDWIN S. GOODRICH. PAGE Crossopterygii ...... 490 Osteolepidre ..... 499 Glyplopomidee ..... 500 Rliizodontidse ..... 500 HoloptychiidfE ..... 501 Ccelacant.liidfE ..... 502 Dipnoi ...... 503 CONCLUSION . .512 INTRODUCTION. IN the following paper are given the results of a study, carried on for some years, on the structure and development of the dermal fin-rays oE fish. The classification of living and extinct fish is a subject which has grown much in importance of late years, and has been greatly advanced in quite modern times through the labours of Cope, Traquair, A. Smith Woodward, and others. Almost every available character has been made use of in turn by systematists in the endeavour to classify the larger groups; and of these cha- racters it is obvious that those which are based on structures capable of being fossilised must be the most useful. No parts of a fish are, as a rule, better preserved than tlie dermal skeleton. Yet, although Agassiz (1) long ago classified fish into large divisions according to the structure of the scales, modern systematists are inclined to attach so little importance to tho exo-skeleton that I venture to think they have somewhat neglected the dermal fin-rays. If it is true that the application of cut and dried definitions of the scaling will no longer enable us satisfactorily to subdivide the Pisces into Placoidei, Ganoidei, Cycloidei, and Ctenoidei; if it is true that "placoid" scales may be present in the Ganoidei, and that various forms of cycloid scales may have been independently evolved from rhomboid ganoid scales in later groups,—nevertheless these facts do not by any means prove that the scaling is of little systematic importance. THE DERMAL FIN-RAYS OP FISHES. 467 Indeed, are there any characters against which the samo objections may not be urged ? Paradoxical as it may seem, yet the most superficial structures are often of the deepest significance in the study of phylogeny. As a mammal may be surely identified by its hair, and a bird by its feather, so may a fish be classified by its dermal fin-ray. The dermal rays, however, are of even more constant structure than the body-scales. There seem to be only three main types of these rays, and it was with the object of defining these types, and of tracing out their phylogenetic relations, that this work was undertaken, in the hope of thereby helping to clear up some obscure points in classification. Unfortunately the task has proved far less easy than I expected, chiefly on account of the many diffi- culties encountered in the study of fossil forms. The work remains incomplete; many important questions are left undecided ; yet I hope enough has beeu done to show that it is a fruitful field for research, from which much may be expected. The bulk of this work was carried out on the material in the Department of Comparative Anatomy of the Oxford Museum. I have to thank Professor Sollas for the loan of specimens from the Geological Department, and Professor Zittel for permission to freely examine the famous Munich collections. More especially I am indebted to Dr. R. H. Traquair and Dr. A. Smith Woodward for much help and advice whilst working through the collections in the Edin- burgh and British Museums. The dermal rays of fish have already been described in considerable detail by many authors. In the first half of the nineteenth century Agassi:?, in his classical memoir (1), mentioned the bony and generally jointed rays of the higher fish, and the unjoin ted horny rays of the Selachians and Chimffira. He also described the slender horny rays in the embryonic fins of Teleostean fish, and the dermal rays of many fossil forms. Bruch in 1861 (5) first drew a clear distinction between the endo-skeletal rays and the secondary 468 EDWIN S. GOODBIOH. rays belonging to the dermal skeleton. 0. Hertwig, in a series of most important papers on the dermal skeleton of fish (17), contributed numerous observations on the structure and development of the rays, and clearly established the homology of the jointed bony rays of the Actinopterygii -with the body-scales. Beaudelot (2) had previously put forward the same view, and had given a clear general account of the development of the dermal rays in Teleosts, which subject has again been most successfully studied in detail by Harrison (13). An interesting general discussion of the dermal rays has been published by Ryder (32), in which he upholds the view that they are homologous throughout all the groups of fish in which they occur. Pander (27 and 28), Traquair (35, 36, 37), A. Smith Woodward (42), and others have from time to time contributed many important observa- tions on the fin-rays of extinct forms. Before describing the dermal fin-rays of the true Fish, it may be mentioned that real dermal rays are absent in Amphioxus and the Cyclostomes. The ray-like structures figured and described by various authors in the larva of Amphioxus (Hatschek [16], Lankester and Willey [23]) are elongated epidermal cells. They have nothing to do with the dermal rays of fish. In the Cyclostomes, on the other hand, the fins are supported by delicate cartilaginous rays, prolonga- tions of the neural and haemal arches of the axial endo- skeleton.1 Elasmobranchii. It is well known that the fins of the Elasmobranch fish, whether paired or unpaired, are provided with so-called " horny fin-rays." These rays are slender, flexible, cylindrical rods of fibrous substance, deposited in concentric layei's. They are pointed at the proximal end, and gradually diminish 1 Dermal rays are figured by mistake in the adult Bolellostoma by B. Dean (8)t It would be interesting to know whether there are any traces of such rays in the young. 1'Hti DERMAL FIN-RAYS 01* WISHES. 469 in diameter towards the distal extremity. The horny fin- rays—which I shall call the ceratotrichia—never become ossified, and are never jointed. Moreover, as a rule, they are very numerous, and form more than one layer, are set close together, are evenly distributed over the fin, and occasionally may branch. In the adult fish, except near the distal grow- ing edge of the fin, they are deeply embedded in the con- nective tissue some way from the epidermis (figs. 1 and 7 ct.), and overlap to a considerable extent the cartilaginous fiu- supports. They are developed on both sides of the fin, and proximally embrace the median cartilaginous rays. The proximal ends of the ceratotrichia are enclosed in the sheet of connective tissue to which the special fin-muscles are attached (fig. 1, cnt.). These muscles, developed from the muscle-buds which sprout from the myotomes, are the appendicular muscles which occur in the fins of all fish, and are parallel to and correspond with the endo-skeletal rays. Outside the ceratotrichia and the surrounding layer of con- nective tissue lie the denticles, and lastly the epidermis. The denticles bear no relation either in position or in number to the underlying dermal rays. Klaatsch (21) has given some details concerning the early development of the ceratotrichia. According to him cells derived from the epidermis migrate inwards into the under- lying mesoblastic tissues, where they secrete the rays. The horny substance appears first in the centre of these " sclero- blasts," and subsequently enlarges and becomes surrounded by a multicellular sheath. These statements are supported by very unconvincing figures. My own observations are directly opposed to Klaatsch's interpretation. Sections through embryos of Scyllium or Acanthias properly preserved, and treated with appropriate stains (such as borax-carmine and picro-nigrosin), show conclusively that a continuous and definite basement mem- brane separates the epidermis from the mesoblastic tissues over the whole surface of the animal (figs. 7—12 b.m.). At what stage this membrane first makes its appearance I am 470 JJDWIN S. G00DR10H. unable to say; but there can bo no doubt that it is formed very early indeed. It is clearly present, for instance, in Scyllium embryos 14 mm. in length, long before either denticles or dermal rays have begun to develop. Once laid down the basement membrane remains, as far as can be seen, continuous and unbroken throughout embryonic life. Of any migration of scleroblasts from the epidermis there is not the slightest trace, and I have no hesitation in stating that it does not occur in the embryos I have examined.1 After the deposition of the basement membrane, which stains dark blue with nigrosiu, the actinotrichia begin to develop in those regions of the, at first, continuous median fin which are destined to give rise to the adult fins. Almost simultaneously ceratotrichia begiu to develop in the paired fins. Since the fins grow at their free edge, the same section shows various stages in the development of the rays according as the parts nearer or farther from the body are examined. Near the edge itself the first origin of the rays can be traced.
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