56o 



STRUCTURE OF VERTEBRATES 



two or three, on each side, and no sort of attachment to the verte- 

 bral column. In the dogfish (Figs. 241, 242) the cartilages of each 

 side meet and fuse ventrally in both girdles. The names given 

 to the portions of the girdles, or to their separate elements, are 

 descriptive only, and must not be taken to imply homology. 

 The modern elasmobranchs have lost all trace of exoskeleton 

 except in so far as the dermal denticles are homologous with the 

 armour of the early vertebrates (see p. 544). but the bony fishes 



have not only a series of 

 superficial bones in the skull, 

 but a dermal shoulder girdle 

 as well. It is attached 

 dorsally to the temporal 

 region of the skull, and runs 

 down in a half-hoop behind 

 the gill shts ; its functional 

 importance is that where the 

 gill slits are large and placed 

 close together the body-wall 

 in the pharyngeal region is 

 very much weakened. The 

 dermal girdle then fastens 

 the head to the rest of the 

 body. It is seen at about its 

 maximum in the primitive 

 actinopterygian Polypterus. 

 The only bone not present 

 here is the interclavicle, a 

 median ventral piece which 

 often unites the clavicles of the two sides. In the teleosts (Fig. 436) 

 the cleithra meet ventrally and the clavicles are generally absent. 

 In swimming animals the girdles are mere articulating points 

 on which the fins can be turned in their functioning as elevators, 

 but in land animals they come more and more to bear the 

 creature's weight as the body is lifted off the ground. In con- 

 sequence they become progressively strengthened and more 

 firmly attached to the backbone. The change, however, is a 

 gradual one. A young newt when put on land uses the muscles 

 all the way down the body, and throws its whole length into 

 waves similar to those seen in a swimming fish. It is probable 

 that the earliest Stegocephalia did no better ; their heavy bodies 



Fig. 436. — The right half of the pectoral 

 girdle and right pectoral fin of a cod.— 

 From Reynolds. 



I, Post-temporal; 2, supracleithrum ; 3, cleithrum ; 

 4, coracoid ; 5, scapula ; 6, post-cleithrum ; 

 7, brachial ossicles ; 8, dermal fin rays. 



