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Show 54 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, Gegenbaur has most ingeniously suggested that the pectoral arch, with its limb, would correspond with a branchial arch and its rays. It will be observed that the view of the special homologies of the elements of the skeletons of the fins of fishes which I have ventured to put forth differs, fundamentally, both from that suggested by Dr. Gunther and from that advanced by Gegenbaur, either in its original form or as he has modified it subsequently to the discovery of Ceratodus. The former says (I. c. p. 5 3 3 ) : - " W h e n I designated the arrangement of the parts of this pectoral skeleton unique, I did not mean to convey the idea that no homological relation could be pointed out between the parts of the pectoral skeleton of Ceratodus and that of other fishes. It is quite evident that we have here a further development of the simple pectoral axis of Lepidosiren in the direction towards the Plagiostomes. The pectoral skeleton of Lepidosiren paradoxa consists merely of the central series of cartilages of Ceratodus ; there is no fin-like expansion of the skin of the pectoral limb, which is a simple tapering filament. In Lepidosiren annectens this pectoral filament is bordered by an expansion of the skin along its lower edge; and even minute fin-rays are imbedded in each lamina of the fold ; in order to support this low, one-sided, rayed fringe, very small, single-jointed cartilages are added to the axis*. The fin is still more developed in Ceratodus: it has become a broad, scythe-shaped paddle, dilated by a fold of the skin, with two layers of fin-rays surrounding it in its entire circumference ; therefore supporting cartilaginous branches are added on both sides of the axis ; and most of the branches are composed of several joints, in order to reach the more distant parts which require the support." This is the exact converse of the view of the relations of Lepidosiren and Ceratodus which, in agreement with Gegenbaur, I am disposed to take. The fin of the former appears to me to be a reduced and metamorphosed state of the more primitive condition retained in Ceratodus. Dr. Gunther goes on to say that "the arrangement of the limb-skeleton of Ceratodus is foreshadowed in the pectoral fin of Acipenser." On the contrary, in m y judgment, the pectoral fin of Acipenser has been derived by much modification from a Ceratodus-Uke type. In referring to those points in which I venture to dissent from Professor Gegenbaur's interpretation, I cannot refrain from expressing m y sense of the very great value of his investigations into the morphology of vertebrate limbs, and m y grateful indebtedness to the rich fund of new facts and new ideas which they contain. However, I found myself unable fully to accept his theory of the fish's fin and the vertebrate limb generally, in its original form ; and I expressed m y hesitation and its grounds in the German version of my 'Manual of the Vertebrata'f. Gegenbaur's later view is con- * Four or five of these ray-bearers are obliquely attached to each joint of the axis (Peters, Midler's ' Archiv,' 1845, Taf. 2. fig. 2). t ' Handbuch der Anatomie der Wirbelthiere,' iibersetzt von Dr. F. Eatzel (Breslau, 1873), pp. 34, 35. |
