THE ANIMAL KINGDOM 1 7 



of the latter are protected by fetal membranes, while those of the former 

 are without them. 



Class Reptilia 



Reptiles are horny-scaled vertebrates which breathe by lungs. The 

 embryos develop in a liquid-filled sac, the amnion. The skull articulates 

 with the atlas vertebra by means of a single occipital condyle. Arterial 

 and venous blood are mixed in the dorsal aorta. 



Living reptiles are divided into Rhynchocephalia, Lacertilia, Ophidia 

 (Serpentes), Chelonia, and Crocodilia. Among fossil orders, the Thero- 

 morpha are important, since, especially in their dentition, they resemble 

 mammals, and the dinosaurs because they are the ancestors of the birds. 



Rhynchocephalia are mostly fossil reptiles having very primitive char- 

 acteristics. Like some primitive amphibians they have only two sacral 



SPHENODON 



Fig. 19. — Sphenodon has been characterized as a "living fossil." Asa "primi- 

 tive" type of reptile it interests the student of phylogenesis. It belongs to the Order 

 Rhynchocephalia. 



vertebrae. Sphenodon (Hatteria), the only living representative, lacks 

 the external copulatory organs present in all other reptiles. 



The Lacertilia are the lizards. They usually have two pairs of limbs; 

 the anus is a transverse slit; eyelids and an external ear opening are usually 

 present. 



Ophidia (Serpentes) are limbless reptiles devoid of movable eyelids 

 and external ear-opening; the tongue is forked; the scales along the ventral 

 side of the body are specially modified to assist in locomotion. Snakes. 



Chelonia (Testudinata) are toothless reptiles, the broad bodies of which 

 are enclosed by a "shell" which consists of a dorsal carapace and a 

 ventral plastron. The eyes have lids and nictitating membrane. Turtles 

 and tortoises. 



Crocodilia have their teeth set in alveoli; the anus is a longitudinal slit; 

 the tail is laterally compressed; the bodies are large. Alligators and 

 Crocodiles. 



Theromorpha are fossil reptiles which may have been the progenitors of 

 mammals; in some the teeth are differentiated as in mammals; the quadrate 



