REPTILES. 



343 



The body is more or less completely covered with scales, 

 and the toes, when present, bear 

 claws. The scales differ from 

 those of fishes in being outside 

 of the outer layer of the skin. 

 These scales differ much in ar- 

 rangement, etc. The large plates 

 covering the carapace of the tur- 

 tle are but enlarged scales, while 

 the bony armor of the alligator is 

 composed of scales, rendered more 

 protective by the development of 

 bone in the deeper layer of the 

 skin. In the snakes the scaly 

 covering is periodically shed. 



By the greater development of 

 the neck the heart is carried back 

 to a greater distance from the 

 head than in the Batrachia. In 

 all except the alligators the heart 

 is three-chambered, and in these 

 the ventricle is incompletely di- 

 vided into two. There are two 

 aortic arches (fig. 141), but the 

 left one, which also supplies the 

 stomach, is smaller where it joins 

 its fellow to form the dorsal aorta. The blood is 'cold,' 

 or rather it is variable in temperature, varying with that 

 of the air or water in which the animal lives. 



The brain is small, no part being extremely developed, 

 and the optic lobes touch, or may touch, each other in the 

 median line. In snakes, lizards, and turtles the cerebel- 

 lum is small; in the alligators it is larger. 



FIG. 141. Arterial Circulation 

 of Turtle. a, right aortic 

 arch; b, bronchus; /, artery 

 to fore limb ; h, artery to 

 hind limb; p, pulmonary 

 artery; r, renal arteries; 

 s, arteries to stomach; t, 

 trachea; 1, 2, 4, persisting 

 aortic arches. Compare 

 with fig. 117, E. 



