IV. VERTEBRATA. 



and suppression of the intermediate regions. Of these the un- 

 paired are possibly the oldest, since they occur not only in the 

 cyclostomes, but in Ampliioxus and the tunicates as well, where 

 paired appendages are lacking; on the other hand they disappear 

 in the higher forms. Since they are of service only in an aquatic 

 life, they are lost in the Amphibia, in which a continuous fin, un- 

 supported by skeletal elements, occurs only in larval life. On the 

 other hand the paired appendages gain in importance with terres- 

 trial habits. 



In the fins of fishes two kinds of skeletal elements occur which, 

 in the Elasmobranchs, are distinguished by their histological 

 structure, since the one, the fin supports (basalia and radialia), 

 consist of cartilage, the others (actinotrichia, dermal skeleton) 



FIG. 563. Pectoral girdle and left fin of Heptanchus. (After Wiedersheim.) a, prin- 

 cipal row of the cartilaginous fin supports ; /i, horny threads or fin rays cut 

 across at /*'; n.Z, foramen for nerve; ?, accessory cartilaginous fin supports ; 

 s, s', scapula ; w, ventral portion of girdle. 



are of horny consistency (fig. 563). Since in the teleosts both 

 kinds of supports may ossify, the distinction is here less striking, 

 yet the basalia and radialia arise from cartilage and lie in the basal 

 part of the fin, while the others are never cartilaginous and occur 

 in the distal portion. These distinctions are of importance, since 

 the actinotrichial portions play no part in the development of the 

 extremities of the higher groups. These arise from the basal sup- 

 ports of pectoral and pelvic fins, which therefore alone need further 

 mention. 



The skeleton of the paired appendages, preformed in cartilage, 

 consists of two parts, the girdles lying in the lateral walls of the 

 trunk, and the skeleton of the limbs themselves. A girdle a 

 shoulder or pectoral girdle in front, a pelvic girdle for the hind 



