302 



CLASSIFICATION OF VERTEBRATES. 



radius and ulna or tibia and fibula, while the carpales and tar- 

 sales in the same way become associated with the metacarpals 

 or metatarsals, thus producing an intracarpal or intratarsal joint. 

 The modifications of metacarpals, metatarsals, and phalanges are 

 more varied ; and we may have walking-feet, as in most reptiles, 

 swimming-feet or paddles, as in ichthyosaurs, 

 plesiosaurs, pythonomorphs, and some turtles, 

 or, as in the pterodactyls, the anterior pair 

 may be modified into supports for the organs 

 of flight. In the swimming-feet there is fre- 

 quently a reduction in length of the proximal 

 bones, while the number of phalanges may be 

 indefinitely increased. 



The heart is farther removed from the 

 head than in the ichthyopsida, and the sinus 

 venosus becomes connected with the right au- 

 ricle. Into the sinus empty the post- and the 

 two precavae, except in the ophidia, where 

 the left precava opens directly into the au- 

 ricle. The greatest advance is seen in the 

 development of a partial or complete (croco- 

 diles) septum, dividing the ventricle into right 

 and left halves. Even when the septum is 

 incomplete the ventricle is actually divided 

 in contraction, the right side containing only 

 venous blood, while the left receives that re- 

 turning from the lungs. Associated with the 

 division of the ventricle is a corresponding 

 division of the ventral aorta of the ichthy- 

 opsida into three trunks, two connected with 

 the right and one with the left ventricle. One 

 of those arising from the right ventricle forms the pulmonary 

 artery, blood passing through it, by means of the last aortic 

 arches, to the lungs. The other right ventricular trunk con- 

 nects by means of the fourth arch of the left side with the left 

 aortic root. Thus, as will be seen, venous blood is forced from 

 the right ventricle of the heart into the lungs and into the 

 dorsal aorta. The aortic trunk arising from the left ventricle 



FIG. 299. Arte- 

 rial trunks of turtle 

 (Emys), after Wied- 

 ersheim. a, left aor- 

 tic arch; 6, bronch- 

 us; /, to fore limbs; 

 h, to hind limbs ; r, 

 renals; s, to stomach. 



