xii PHYLUM CHORDATA 431 



CLASS IV, REPTILIA, 



The class Reptilia comprises the Lizards and Snakes, 

 the Tuataras, the Turtles and Tortoises, and the Alligators 

 and Crocodiles. On a superficial comparison of these with 

 the Amphibia, it might be inferred that there is a close 

 alliance between the two groups ; but this impression 

 becomes weakened when a closer examination is made of 

 their structure and development, and it at length becomes 

 evident that in the Reptilia we have to do with a class of 

 Vertebrates which stand on a distinctly higher plane than 

 the Amphibia. One significant feature of the Reptilia 

 which marks them off sharply from the Amphibia is that 

 the lungs are the sole organs of respiration, gills never being 

 developed at any stage. Another is the development in the 

 embryo of two structures known as the amnion and the 

 allantois, not developed in lower groups of Vertebrates, but 

 present in the embryos of all the higher. The amnion is 

 a thin membrane which covers over the body of the embryo, 

 the space between it and the latter being tensely filled with 

 a watery fluid. The amnion thus forms a sort of water- 

 cushion, protecting the delicate and fragile embryo from 

 the effects of any shocks which may be sustained by the 

 eggs. The allantois, represented in the Frog by the urin- 

 nary bladder, is a membranous structure developed as a 

 hollow outgrowth of the enteric canal at its posterior end. 

 It becomes highly vascular, and acts as an embryonic 

 respiratory organ. 



There are four well-marked orders of living Reptiles : 

 i. The Squamata, comprising the Lacertilia or Lizards 

 (including the Iguanas, Monitors, Skincs, Geckos, Cham- 

 seleons, and others), and the Ophidia or Snakes (including 



