CHAPTER XXIII. 



REPTILES. 



Class Reptilia. 



The three highest classes of Vertebrates — Reptiles, Birds, 

 and Mammals — are often distinguished as Amniota from 

 Amphibians and Fishes, which are called Anamniota. 

 These terms refer to the fact that the embryos of the three 

 higher classes possess a very characteristic birth-robe — the 

 amnion — with which another — the allantois — is always asso- 

 ciated. It seems that the cloacal bladder of Amphibians is 

 homologous with the allantois, but it is not utilised in 

 embryonic life. We shall afterwards explain the nature of 

 the amnion and the allantois, but meantime the possession 

 of these may be recognised as characteristic of Reptiles, 

 Birds, and Mammals. It is also important to notice that no 

 Vertebrates above Amphibians breathe by gills even in early 

 life, a fact in part explained by the development of the 

 allantois, which secures the respiration of the embryo. 



Of these three highest classes the first two are closely 

 linked to one another. Different as they are in habit and 

 external form. Reptiles and Birds are united by deep struc- 

 tural resemblances. These were first clearly recognised by 

 Huxley, when he united the two classes as Sauropsida in 

 contrast to Mammalia on the one hand, and Ichthyopsida 

 (Amphibians and Fishes) on the other. But though this 

 classification is very important, it must not be allowed to 

 obscure the fact that Reptiles possess affinities with both 

 Amphibians and Mammals. Let us, however, state some of 

 the characters which Huxley recognised as distinctive of 

 Sauropsida in contrast to Ichthyopsida and Mammalia. 



