IV. VERTEBRATA 



463 



ported by skeletal elements, occurs only in larval life. On the other hand 

 the paired appendages (arms and legs) gain in importance with terrestrial 

 habits. 



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

 Elasmobranchs (fig. 520), are distinguished by their histological structure, 

 since the one, the fin supports {basal i a and radial i a), consist of cartilage, the 

 others {actinotrichia, dermal skeleton) are 

 of horny consistency. Since in the teleosts 

 both kinds of supports may ossify, the dis- 

 tinction is here less striking, yet the basalia 

 and radialia arise from cartilage and lie in 

 the basal part of the fm, while the others 



Fig. 520. Fig. 521. 



Fig. 520. — Pectoral girdle and left fin of Heptanchus (after Wiedersheim). a, 

 principal row of radialia; h, actinotrichia cut across at h'\ nl, foramen for nerve; r, 

 radialia; s, s', scapula; u, coracoid portion of girdle; i, 2, 3, basalia; i, propterygium; 2, 

 mesopterygium; 3, inetapter^'gium. 



Fig. 521. — Right half of shoulder girdles of {A) frog, (B) turtle, (C) lizard (after 

 Gegenbaur, slightly modified), cl, clavicle; co, coracoid; e, episternum; j, scapula; s', 

 suprascapula; st, sternum, in C with bases of ribs. 



are never cartilaginous and occur in the distal portion. These dis- 

 tinctions are of importance, since the actinotrichial portions play no 

 part in the development of the extremities of the higher groups. These 

 arise from the cartilage supports of pectoral and pelvic fins, which there- 

 fore alone need further mention. 



The skeleton of the paired appendages, preformed in cartilage, con- 

 sists 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 limbs — is in its simplest form 

 an arch with right and left halves, each half with an articular surface for 



