124 



COMPABATIVE ANATOMY 



-P 



.^flf/ 



-F^ 



intermediate ray reaches the arch : this condition maybe compared 

 with that seen in the highly-developed pectoral fin of folypterus 

 (comp. Fig. 103). 



The form of the pelvic fin in bony Ganoids may be easily 

 derived from that seen in the cartilaginous representatives of this 

 Order, but the number of radii is greatly reduced (Fig. 87). The 



rays supporting the distal part of 

 both pairs of tins are bony (comp. 

 p. 103). 



Teleostei. — A still further reduc- 

 tion has taken place in the primitive 

 skeleton of the paired fins in Tele- 

 osts, there being at most only a few 

 radials articulating with the arch 

 (Fig. 84), and even these (especially 

 in the case of the pelvic fin) may be 

 wanting. The main part of each fin 

 is supported by bony rays, as in 

 osseous Ganoids. The skeleton of 

 the fins of Siluroids, Cyprinoids, 

 and Gymnotidse comes nearest to 

 that of Ganoids. 



Phylogeny of the Ichthyoj^terygium. 



Two essentially different views 

 exist as to the primitive form of 

 fin-skeleton in Fishes. As already 

 mentioned on p. 106, Gegenbaur 

 postulates a biserial fin as the primi- 

 tive type (archipterygium), which 

 is most clearly retained in Cera- 

 todus. He supposes that the 

 uniserial form has been derived 

 from this by a reduction of the rays 

 on one side and a further develop- 

 ment of those on the other. The 

 axial ray of the biserial fin would 

 thus correspond to the basi- or 

 metapterygium, while the pro- and 

 mesopterygia of the uniserial fin would answer to special develop- 

 ments of the proximal ends of certain of the rays on one side of it. 

 The other view, which seems a far more probable one, is that 

 the uniserial type is the more primitive, and that this type is most 

 nearly retained in Elasmobranchs, which are as ancient a group as 

 the Dipnoans and which have not passed through a Dipnoan stage 

 in the course of their phylogenetic development. 



Fig. 82 represents the mode of origin of the Elasmobranch fin 



Fig. 103. — Pectoral Fin of 

 Polypterus. 



Pr, 



Ms, Ml, pro-, meso-, and meta- 

 pterygium, the first and last 

 mentioned meeting at f, so that 

 the mesopterygium does not 

 reach the arch ; OSS, centre 

 of ossification in MS ; * part of 

 the mesopterygium which ex- 

 tends between the distal end of 

 the propterygium and the first 

 row of radii ; Nl, nerve for- 

 amina in the mesopterygium ; 

 Ra, Ra^, radii ; FS, bony der- 

 mal rays. 



