The Skeleton of the Fish 



the back-bone at a distance behind the head. To this cartilage 

 three smaller ones are attached, forming the base of the pectoral 

 fin. These are called mesopterygium, protcrygiwn, and meta- 



pterygium, the first named 

 being in the middle and 

 more distinctly basal. 

 These three segments are 

 subject to much varia- 

 tion. Sometimes one of 

 them is wanting; some- 

 times two are grown to- 

 gether. Behind these the 

 fin-rays are attached. In 

 most of the skates the 

 shoulder-girdle is more 

 closely connected with 

 the anterior vertebrae, 

 which are more or less 

 fused together. 



The pelvis, remote 

 from the head, is formed, 

 in the shark, of a single 

 or paired cartilage with 

 smaller elements at the 

 base of the fin-rays. In 

 the males a cartilaginous 

 generative organ, known 

 as the clasper, is attached 

 to the pelvis and the 

 ventral fins. In the 

 Elasmobranchs the tail 

 vertebras are progressively smaller backward. If a caudal fin 

 is present, the last vertebrae are directed upward (heterocercal] 

 and the greater part of the fin is below the axis. In other forms 

 (sting-rays) the tail degenerates into a whip-like organ (lepto- 

 cercal), often without fins. In certain primitive sharks (Ichthyo- 

 tomi), as well as in the Dipnoi and Crossopterygii, the tail is 

 diphycercal, the vertebrae growing progressively smaller back- 

 ward and not bent upward toward the tip. 



FIG. 47. Shoulder-girdle of a Flounder, Para- 

 lichthys californicus (Ay res). 



