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Show 1881.] OF THE PAIRED FINS OF ELASMOBRANCHS . 661 exhibits may be derived from a ground-form which persists in a few cases only, and which represents the first, and consequently the lowest, stage of the skeleton in the fin-the archipterygium. This is made up of a stem which consists of jointed pieces of cartilage, which is articulated to the shoulder-girdle and is beset on either side with rays which are likewise jointed. In addition to the rays of the stem there are others which are directly attached to the limb-girdle. " Ceratodus has a fin-skeleton of this form ; in it there is a stem beset with two rows of rays. But there are no rays in the shoulder-girdle. This biserial investment of rays on the stem of the fin may also undergo various kinds of modifications. Among the Dipnoi, Protopterus retains the medial row of rays only, which have the form of fine rods of cartilage ;- in the Selachii, on the other hand, the lateral rays are considerably developed. The remains of the medial row are ordinarily quite small, but they are always sufficiently distinct to justify us in supposing that in higher forms the two sets of rays might be better developed. Rays are still attached to the stem and are connected with the shoulder-girdle by means of larger plates. The joints of the rays are sometimes broken up into polygonal plates which may further fuse with one another; concrescence of this kind may also affect the pieces which form the base of the fin. By regarding the free rays, which are attached to these basal pieces, as belonging to these basal portions, we are able to divide the entire skeleton of the fin into three segments-pro-, meso-, and metaptery-gium. " The metapterygium represents the stem of the archipterygium and the rays on it. The propterygium and the mesopterygium are evidently derived from the rays which still remain attached to the shoulder-girdle." Since the publication of the memoirs of Thacker, Mivart, and myself a pupil of Gegenbaur's, M . v. Davidoff **, has made a series of very valuable observations, in part directed towards demonstrating the incorrectness of our theoretical views, more especially Thacker's and Mivart's view of the genesis of the skeleton of the limbs. Gegenbaur2 has also written a short paper in connexion with Davidoff's memoir, in support of his own as against our views. It would not be possible here to give an adequate account of Davidoff's observations on the skeleton, muscular system, and nerves of the pelvic fins. His main argument against the view that the paired fins are the remains of a continuous lateral fin is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united by a longitudinal commissure with the true plexus of the nerves supplying the fin. Prom this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the 1 M. v. Davidoff, " Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische, I.," Morphol. Jahrbuch, vol. v. 1879. 2 " Zur Gleidmaassenfrage. An die Untersuchungen yon Davidoffs ange-knupfte Bemerkungen," Morphol. Jahrbuch, vol. v. 1879. |