406 



COLLEGE ZOOLOGY 



both species and individuals. About 243 

 species of all the reptiles that have been 

 described are known to occur in this coun- 

 try. Reptiles occupy an important place in 

 the vertebrate series because their anatomy 

 is intermediate between that of a typical 

 amphibian on the one hand and that of a 

 typical bird on the other. Comparison of the 

 structure and physiology of reptiles with 

 those of amphibians and birds is well worth 

 while. The poisonous nature of certain rep- 

 tiles, the enormous size of some of the pre- 

 historic species, and the relations of reptiles 

 to man are among the most interesting gen- 

 eral features of the class. 



TURTLE 



The turtle has been selected as a repre- 

 sentative reptile. The body is so constructed 

 that it is adapted to live either in the water 

 or on land. Although it is slow-moving on 

 land, it can swim quite rapidly. 



External features 



The turtle is distinguished from all other 

 animals by the shell which is broad and 

 flattened and protects the vital organs (Fig. 

 275). Even the head, limbs, and tail can be 

 more or less completely withdrawn into the 

 shell. The neck is long and very flexible; the 

 head is flattened dorsoventrally. The mouth 

 is large, but instead of teeth, horny plates 

 form the margin of the jaws; they are used 

 to crush their food. The external nares (nos- 

 trils) are placed together, near the anterior 

 end of the snout. The eyes, situated one on 

 each side of the head, are each guarded by 

 three eyelids: (1) a short, thick, opaque 

 upper lid; (2) a longer, thin lower lid; and 

 (3) a transparent nictitating membrane, 

 which moves over the eyeball from the an- 

 terior corner of the eye. Just behind the 

 angle of the jaw on either side is a thin 

 tympanic membrane. The limbs usually 

 possess 5 digits each; most of the digits are 

 armed with large horny claws that are useful 



in crawling, climbing, or digging. The skin 

 is thin and smooth on the head, but thick, 

 tough, scaly, and much wrinkled over the 

 exposed parts of the body. 



Skeleton 



Since the life of the turtle is influenced so 

 strongly by the skeleton, this system will be 

 briefly described first. 



The shell (Fig. 275) consists of a convex 

 dorsal armor, the carapace, and a flattened 

 ventral armor, the plastron; these are 

 strongly bound together on each side by 

 bony bridges varying in width with the 

 species. Both carapace and plastron are usu- 

 ally covered by a number of symmetrically 

 arranged horny plates, called scutes 

 (shields); the scutes do not correspond 

 either in number or arrangement to the 

 bony plates beneath them. The number and 

 shape of the scutes vary according to the 

 species but are usually constant in individ- 

 uals of the same species. The horny scutes 

 of the hawksbill turtle furnish the tortoise 

 shell of commerce. Beneath the scutes are a 

 number of bony plates formed by the dermis 

 and closely united by sutures. 



The vertebrae and ribs are usually con- 

 solidated with the bony carapace; no ster- 

 num is found in these forms. Soft-shelled 

 turtles (Fig. 280) have a leathery shell 

 which is not divided into scutes, and it con- 

 tains little bony substance. 



Digestive system 



Turtles feed on both plants and animals; 

 some are entirely vegetarian. The animals 

 preyed upon are waterfowl, small mammals, 

 and many kinds of invertebrates. The flex- 

 ible neck enables the turtle to rest on the 

 bottom and reach out in all directions for 

 food. The jaws of large snapping turtles are 

 powerful enough to amputate a finger, or 

 even a hand. 



The digestive organs are simple. The 

 broad soft tongue is attached to the floor of 

 the mouth cavity; it is not protrusible. The 



