CHAPTER XXIV. 



REPTILES. 



Classes — Chelonia. Rhynchocephalia. Lacertilia. 

 Ophidia. Chelonia, and Extinct Classes. 



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

 dilians, &c. — 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 em- 

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

 Reptiles are essentially creatures of the earth, but many 

 lizards, snakes, and turtles, and all the crocodilians, are 

 aquatic. Partially marine forms are represented by the 

 Galapagos lizard, which swims out among the seaweed, by 

 some crocodilians which venture down the estuaries ; some 

 turtles live far out to sea and only seek the shores to 

 lay their eggs ; the Hydrophids, or sea-snakes, never leave 

 the water. 



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 

 a characteristic protective foetal membrane — the amnion — 

 with which the allantois is always associated. 



Of these three highest classes of Vertebrates, the Reptiles 

 and Birds, so different in form and habit, are united by deep 

 structural resemblances. These were first clearly recognised 



