CHAPTER LVII 



CLASS REPTILIA 



All reptiles possess three regions of the body — head, trunk, and tail. 

 There is generally a sufficient constriction behind the head to form a 

 neck. The typical body covering is composed of overlapping epidermal 

 scales which form a hard coat of mail, protecting the body against drying 

 and also against injury from ordinary mechanical contacts. Well- 

 developed eyes are present, protected by lids in all forms except the 

 snakes; two nostrils are situated toward the end of the snout; and behind 



Right 



ccfroh'd 



artery 



Right 

 aortic 



Left 



carotid 



artery 



Let^ 

 subcla vian 



artery 



'Pulmonary 

 artery 



Left aortic 

 arch 



Left 

 auricle 



Right 

 auricie 



Ventricle 



Aorta 



Rigtit 

 auricle 



Openings 



from 

 auricles 



Aorta 



Left 

 ciuricle 



B 



Fig. 272. — Reptilian hearts. A, heart and associated blood vessels of snapping turtle, 

 viewed from in front. From a specimen, but somewhat diagrammatic. B, similar repre- 

 sentation of crocodile's heart. {From Hertwig and Kingsley, "Manual of Zoology," by 

 the courtesy of Henry Holt & Company.) Neither figure shows the venae cavae or the 

 pulmonary veins. In both figures the vessels carrying arterial blood are unshaded; those 

 carrying venous blood are black; and those carrying mixed blood are crosslined. The heart 

 of the crocodile is cut open to show the chambers; the direction of blood flow is shown by 

 arrows and the connection between the two aortic arches is also shown. 



each eye is an ear opening except in the snakes. The mouth is terminal 

 and the jaws may bear teeth, as in most reptiles, or may be furnished with 

 a horny beak, as m the turtles. A cloacal opening marks the posterior 

 end of the trunk region proper and the beginning of the tail. Four limbs 

 are generally present, lacking only in snakes and a few lizards. 



413. Classification. — The class Reptilia is divided into four orders: 

 1. Squamata (skwa ma' ta; L., squamatus, scaly). — Chameleons, 



lizards, and snakes. 



386 



