434 MANUAL OF ZOOLOGY SECT. 



at length becomes evident that in the Reptilia we have 

 to do with a class of Vertebrates which stand on a dis- 

 tinctly 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 struc- 

 tures known as the amnion and the allantois, not devel- 

 oped 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, repre- 

 sented in the frog by the urinary 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 : 



1. The Squamata, comprising the Lacertilia or lizards 

 (including the iguanas, monitors, skinks, geckos, chamaeleons, 

 and others), and the Ophidia or snakes (including the vipers 

 and rattlesnakes, pythons, boas, sea-snakes, etc.). 



2. The Rhynchocephalia, including only the New Zealand 

 Tuatara (Hatteria). 



3. The Chelonia, including the land tortoises, soft tor- 

 toises, river and marsh tortoises, and the turtles. 



4. The Crocodilia, including the crocodiles, gavials, the 

 alligators and caimans. 



The most striking external difference between a typical 

 lizard (Fig. 207) and the frog are in the covering of scales 

 in the case of the former, the comparative smallness of its 



