578 TEXTBOOK OF ZOOLOGY 



does not fertilize the eggs of the year but those of succeeding years. 

 In experimenting upon this phase of the life history of turtles, the 

 United States Fish and Wildlife Service has discovered the surprising 

 fact that a female turtle may lay fertile eggs for as long as four 

 successive years after a single mating. 



External Structure 



No vertebrate is so unique in structure as the turtle. No turtle 

 could be mistaken for any other animal. The arched or dome- 

 shaped dorsal covering, the carapace, and the plane floor of this 

 armor, the plastron, set the turtles apart from all living vertebrates. 

 The head and neck can be completely withdrawn into the shelter 

 of this covering; and legs, feet, and tail can be partially protected 

 in this manner. 



The body of a turtle may be divided into the following regions: 

 head, neck, trunk, and appendages. 



The head is covered with thin, smooth skin. Various colored lines 

 and patterns -may be present on the skin of the head and neck. 

 Troost's turtle has an oblong red patch on each side of the head and 

 neck unless it has been completely obliterated by black pigment de- 

 posited over it. The neck is relatively long and slender, and re- 

 tractile within the shell by vertical sigmoid flexures. The skeletal 

 elements of the trunk region are completely co-ossified into an im- 

 mobile boxlike structure. This carapace and plastron are covered by 

 scutes of epidermal tissue. These scutes are definite in number and 

 arrangement. Various color patterns are found on or under them. 

 The legs are typical pentadactyl limbs. The toes are united by 

 various degrees of webbing in aquatic forms. The tail is relatively 

 small and short. 



Digestive System 



The organs of the digestive tract are mouth, pharynx, esophagus, 

 stomach, small intestine, large intestine, rectum, cloaca, and anus. 

 Various accessory structures and glands are the cutting edges of the 

 beaklike jaws, the liver and the pancreas. The size and relative 

 length of the various parts of the alimentary canal are correlated 

 with the food habits of the animal. Turtles eating largely vegetable 

 food have intestines many times the length of the body. Those taking 



