APPENDICULAR SKELETON 223 



lobes ('flukes'). All of these are without skeletal supports other than the 

 vertebras. 



CYCLOSTOMATA.— These eel-like animals have the median 

 fins continuous, there being but slight differentiation of caudal, 

 dorsal and anal regions. The skeleton of the fin proper consists of 

 fihform cartilage rods, irregularly arranged (up to four to a somite 

 in Petromyzon), each spht distally in a dichotomous manner. Ante- 

 riorly these rods are not connected with the reduced axial skeleton, 

 but in the tail their bases are fused and the resulting plates are either 

 articulated (Petromyzon) or fused with 

 the united neural and haemal arches 

 (Myxinoids, figure 234). These rods 

 show no traces of division into basal 

 and radial parts as do those of true 

 fishes. 



FISHES have the skeleton of the 

 median fins partly cartilage, _ partly ^^;,, ^^^^'1,, 2^^ .^ 



dermal in origin. In the simplest tilage; /, fin rays; n, notochord; 5, 

 1. . r 1 1 1 1 i- J.-L. spinal cord; 1', ventral cartilage. 



condition of dorsal and anal fins the 



cartilage parts consist of a series of rods, often more numerous than 

 the vertebras., which are partly or wholly embedded between the 

 trunk muscles of the two sides of the body. These rods have various 

 names, pterygophores possibly being the best. In Elasmobranchs 

 each pterygophore is usually divided into a deeper basal and a more 

 superficial radial, the radial often being subdivided, the distal seg- 

 ment then being a marginal. These pterygophores support the 

 actinotrichia which arise in the skin on either side of the fin, their 

 proximal ends embracing the distal ends of the pterygophores. 



In Elasmobranchs the pterygophores remain cartilage; in Teleos- 

 tomes (Acipenser and a few others excepted) they are ossified, there 

 usually being two (sometimes three) elements in each, the one or 

 two basal being called axinosts or interspinals, the distal being a 

 baseost. The number of axinosts and baseosts usually corresponds 

 to that of the vertebrae at either end of the fin, but in the middle 

 they may be more numerous, even up to four to a somite. In 

 Elasmobranchs the basalia often fuse to large plates (fig. 238) and 

 less frequently there is a similar fusion of radial parts. The ptery- 

 gophores often extend over more somites than does the fin itself, 

 possibly the result of reduction of the continuous fin. Only the 



