588 REPTILES. 



The blood driven to the lungs is purified there, and returns by pul- 

 monary veins to the left auricle. Thence it passes through a valved 

 aperture into the left ventricle. Thence it is driven into the right aortic 

 arch. From this the carotids to the head and the subclavians to the 

 fore limbs are given off. These parts of the body thus receive wholly 

 arterial blood from the heart. 



The venous blood returning from the posterior regions may pass 

 through the kidneys in a renal portal system, and thence into the inferior 

 vena cava ; or it may pass through the liver in a hepatic portal system, 

 and thence by hepatic veins into the inferior vena cava ; or some of it 

 may pass directly into the inferior vena cava. The renal portal veins 

 arise from a transverse vessel uniting the two branches of the caudal, but 

 the latter are also continued forward as lateral epigastrics which enter 

 the liver. 



The temperature of the blood is not above that of the surrounding 

 medium. 



In regard to the respiratory system, we should notice that the lungs 

 are invested by pleural sacs as is the case in Mammals. 



The ureters of the kidneys, the vasa deferentia from the testes in the 

 male, the oviducts from the ovaries in the female open into the cloaca, 

 which has a longitudinal opening. 



The eggs, which in size are like those of geese, have a thin calcareous 

 shell, are buried in excavated hollows, and, warmed by the sun, hatch 

 without incubation. 



Of one species of crocodile it is known that the mother opens up the 

 nest when the young, ready to be hatched, are heard to cry from within 

 the eggs. The mothers take some care of the young, which require to 

 be defended even from the appetite of the males. 



Crocodiles are relatively sluggish, and fond of basking passively, 

 sometimes hiding in the mud during the hot season. They are remark- 

 able for the long continuance of growth, which does not seem to have 

 so definite a limit as in most other animals. 



Classification of Crocodilia. 



(a) The true crocodiles, of the genus Crocodilus, occur in Africa, 

 Southern Asia, tropical Australia, Central America, and the West 

 Indies. 



The Indian Crocodile ( C. porosus] may measure about eighteen feet 

 in length, and even larger forms have been recorded. The sacred African 

 crocodile (C. vulgaris) is still formidably common in some of the fresh 

 waters of tropical Africa. 



The eggs and the young are often eaten by a mammal called the 

 Ichneumon, and by a species of lizard. The adults have few enemies 

 except man. They seem to live in friendly partnership with little birds 

 (Phivianus czgypticus), which remove parasites from the body, and in 

 their familiarity almost justify the account which Herodotus gives of 

 their cleaning the reptile's teeth. 



(b] The Alligators, of the genus Alligator, are, with the exception of 

 one Chinese species, confined to North and South America. In North 

 America A. mississippiensis, in South America A. sclerops, are common. 



