xv. 3 EGGS OF CHELONIA 397 



in the herbivorous and carnivorous forms. The edges of the jaws form 

 sharp ridges, covered with a formidable horny beak. 



The similarity of the soft parts of Chelonia to those of other 

 surviving reptiles (which are all diapsids) suggests that the 

 general organization of the group has changed little since the Per- 

 mian. The heart possesses a partly divided ventricle and there are two 

 equal aortic arches (Fig. 244). Respiration is modified by the rigidity 

 of the body wall; the lungs are spongy structures attached to the 

 dorsal surface of the shell, sometimes enclosed in a separate pleural 

 cavity (Testudo). Breathing is mainly brought about by the contraction 

 of the modified abdominal muscles, which function in a manner 

 comparable with that of the mammalian diaphragm, and by means of 

 pumping movements of the pharynx. Some aquatic forms {Emys) also 

 respire by taking water into special vascular sacs, diverticuli of the 

 urodaeum. The metabolism of tortoises is slow and they can remain 

 for long periods without breathing. In temperate climates all species 

 hibernate regularly. 



The kidney is metanephric and the nitrogenous excreta are mainly 

 uric acid, there being a typical subdivision of the cloaca and reabsorp- 

 tion of water to form a solid whitish excretory product (see p. 380). 

 There is a single copulatory organ and the eggs are whitish, with hard 

 or soft shells. Like other aquatic reptiles that lay eggs, turtles all come 

 ashore to breed. Thus the marine Chelone breeds in the West Indies, 

 in the Straits of Malacca, and on the coast of West Africa; they are 

 caught as they come ashore and made into turtle soup. On the Amazon 

 chelonian eggs are (or at least were) so plentiful that large numbers 

 were eaten. The eggs are usually carefully placed in holes made by 

 boring with the tail and scooping with the feet (Emys). The traces of 

 the nest are then covered, often with considerable success. Neverthe- 

 less Bates reckoned that at least 48 million eggs were taken annually 

 on the upper Amazon. The chief enemies of the young are vultures 

 and alligators, and these were presumably the ultimate losers when 

 collection by man began, though the numbers of turtles have also 

 decreased as a result of the human depredations. 



The proverbial slowness of the tortoise is a necessary corollary of 

 its heavy armour, but the nervous organization and behaviour is more 

 complex than is sometimes supposed. The brain shows well-developed 

 cerebral hemispheres, with not only the basal regions but also the 

 pallium quite large. This was therefore probably true also of the 

 earliest reptiles, as it is of amphibians. In the mammals, also derived 

 from cotylosaur ancestors, there has been still further development of 



