CHAPTER XXIV. 



REPTILES. 



Chelonia. Rhynchocephalia. Lacertilia. Ophidia. 

 Crocodilia. Many Extinct Classes. 



The diverse animals — Tortoises, Lizards, Snakes, Croco- 

 dilians, etc. — which are classed together as Reptiles, are 

 the modern representatives of those Vertebrates which first 

 became independent of the water, and began to possess the 

 dry land. While almost all Amphibians spend at least their 

 youth in the water, breathing by gills, this is not necessary 

 for Reptiles, in which embryonic respiration is secured by a 

 vascular foetal membrane known as the allantois. As in 

 still higher Vertebrates, gill-slits are present in the embryos ; 

 but they are not functional, and are without gills. Reptiles 

 seem to form among Vertebrates a great central assemblage, 

 like " worms " among Invertebrates, rather a number of 

 classes than a class, exhibiting affinities not only with Birds, 

 but with Mammals and Amphibians as well. 



Reptiles, Birds, and Mammals are often distinguished, as 

 Amniota, from Amphibians and Fishes, which are called 

 Anamnia, the terms referring to the presence or absence of 

 the protective fcetal membrane — the amnion — with which 

 another, the allantois, is always associated. Among these 

 Amniota the Mammals stand somewhat apart, while the 

 Reptiles and Birds, so different in form and habit, are 

 united by deep structural resemblances. These were first 

 clearly recognised by Huxley, who united Birds and Reptiles 

 as Sauropsida, in contrast to Mammalia on the one hand, 

 and Ichthyopsida (Amphibians and Fishes) on the other. 

 (See Table.) 



