CHAPTER XXIV 
Ciass REPTILIA 
CHELONIA. RHYNCHOCEPHALIA. LACERTILIA. OPHIDIA. 
CrocopiniaA. 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 
a 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. 
