CHAPTER XXIV. 



CLASS REPTILIA. 



CHELONIA. RHYNCHOCEPHALIA. LACERTILIA. OPHIDIA. 

 CROCODILIA. MANY EXTINCT ORDERS. 



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, more like a number of 

 classes than a single class, exhibiting close affinities with 

 Birds and Mammals, and more distant affinities with 

 Amphibians. 



Reptiles, Birds, and Mammals are distinguished, as 

 Amniota, from Amphibians and Fishes, which are called 

 Anamnia, the terms referring to the presence or absence of 

 the protective foetal membrane the amnion with which 

 another, the allantois, is always associated. Among other 

 common characters the following may be noted: the 

 generally terrestrial habit, the absence of gills, the absence 

 of a conus arteriosus, the breaking up of the ventral aorta, 

 the presence of twelve cranial nerves, the importance of the 

 hyo-mandibular gill-cleft. 



