524 



CHORDATA 



This is inleriarsal, occurring between the first and second rows of tarsal 



bones (fig. 587, C). 



Since reptiles lack even transitory gills, the gill slits are completely 



degenerate before the young escapes from the egg. Dermal respiration is 



far less important than with the Amphibia, lungs, as in birds and mammals, 



being the respiratory organs, and in these 

 a progressive development may be fol- 

 lowed. The larynx is followed by a 

 trachea with cartilage supports in its 

 wall, and this either opens directly into 

 the two lungs or divides into two bronchi, 

 which, in Varanus, may divide again 

 inside the lungs. The lungs in the more 

 primitive forms are subdivided only per- 

 ipherally, but in the higher groups the 

 whole is chambered, partitions extending 

 inwards to the intrapulmonary bronchus 

 (fig- 536). 



Since the respiration is entirely 

 pulmonary, the heart is divided into a 

 left arterial and a right venous half, and 

 a corresponding separation of systemic 

 and pulmonary blood-vessels occurs (fig. 

 574). The two auricles are completely 

 separated, while a septum extends into 

 the ventricle, complete in the crocodiles, 

 but not in turtles, lizards, and snakes. 



Yet r in the crocodiles a mixin of 



/>, pulmonary artery; s, subclavians; arterial and venous blood occurs since 



^t^liiSi'S 1 ^ in < he la ^ *fc t* which arise 



blood How); i, 2, 4, arches compara- from both ventricles a communication, 



FIG. 574. Heart of crocodile with 

 arteries (diagram), a 1 , a 2 , right and 

 left auricles; ad, as, right and left 



heart, and the connection (foramen arterial trunk is divided by internal 



LTSie^S^StX p artitiolls into three ". - h - h e 



of the heart. but rarely visible from the exterior. One 



of these arises from the right ventricle, 



carries venous blood, and takes over the fourth arterial arch, which 

 gives off the pulmonary arteries (4, p). A second vessel arises from the 

 left ventricle, is purely^arterial and connects with most of the remaining 

 arterial arches, the first, which gives off the carotid, and the right half 

 (aortic arch, ad) of the second arch. The third vessel connects on the 



