CLASS REPTILIA 389 



being shed at intervals and replaced. The tongue is flat and cannot 

 be protruded, but it may be lowered and carried backward to prevent 

 water from entering the esophagus if the mouth is opened while the 

 animal is submerged. Lateral folds meet and form a palate that separates 

 the nasal chamber from the mouth and the nasal chamber becomes 

 divided by a median septum. The lungs are in a pleural cavity sepa- 

 rated from the rest of the body cavity by a diaphragm analogous to that 

 of the mammals. 



420. Testudinata. — The turtles and tortoises, which names are 

 used interchangeably, show the greatest departure from the typical 

 reptilian form. The body is inclosed in a shell (Fig. 266) consisting 

 of a dorsal carapace and a ventral plastron. Into this shell may be 

 drawn the head and neck, limbs, and tail, when the animal is threatened 

 with danger. Although turtles breathe by means of lungs, they may 

 remain under water for a considerable time before needing to come 

 to the surface for air. Since the shell prevents the lungs from being 

 expanded and contracted, air is pumped into them by movements of 

 the neck and feet. Some aquatic turtles also possess thin-walled sacs 

 on each side of the cloaca which may be alternately emptied and filled 

 with water and through the vascular walls of which respiration may take 

 place while the animal is submerged. 



All turtles are oviparous. Their eggs, which are nearly spherical, 

 are covered with a hard, white shell and deposited in nests in the ground. 



America is the richest of all regions in its turtles and tortoises. Giant 

 tortoises found on islands off the west coast of South America and in 

 the Indian Ocean reach a weight of more than 300 pounds and probably 

 attain an age of over four hundred years. They are relics of past ages 

 and owe their survival to the isolation of the islands on which they live. 

 The largest turtle known is a marine leathery turtle which reaches a 

 weight of 1000 pounds or more. 



421. Economic Importance. — Reptiles are economically either injuri- 

 ous or beneficial. Most snakes, being nonvenomous and destroying 

 injurious insects and mammals, are distinctly beneficial, though some 

 do injury by destroying birds and their eggs and young. Other snakes 

 are dangerous to man because of their venom. Generally speaking, 

 lizards are beneficial because of their insect-eating habits; one or two 

 have been noted as being sources of food; and on the other hand one 

 type, the Gila monster, is poisonous. Crocodiles and alhgators are 

 feared in the countries in which they live because of their attacks upon 

 persons going into the water, but they are valuable for the skins which 

 they furnish. None of the turtles is injurious, and some are useful as 

 food, being more often used in this manner than are any other reptiles. 

 Certain tortoises yield tortoise shell which is widely used commercially, 

 although it is now largely replaced by an artificial product. 



