CHORDATA 31 I 



collectively the Amniota ; in distinction from them the fisl 

 and amphibians are called the Anamnia. As the young of the 



Amniota do not develop in the water, gills would of course be 

 useless, although here is evidence of gill slits ; so another organ 

 of respiration is necessary for the embryo, and this is found in a 

 sort of membranous sac called the allantois, which develops at 

 the posterior end of the alimentary canal. The yolk supplies 

 nutriment to the young animal and is absorbed through a small 

 region in the ventral body wall called the umbilicus. 



The turtles are most abundant in the temperate regions, 

 while other reptiles are most numerous in the tropics, and 

 some, such as the crocodiles and their allies, are essentially 

 tropical in habitat. The reptiles through their fossil repre- 

 sentatives show very close relationships to the birds through 

 the fossils of the latter ; so that zoologists frequently divide 

 the Vertebrata into three groups: the Ichthiopsida(Gr. t^flw, fish, 

 and o-v/ri?, appearance), including the fishes and amphibians ; the 

 Sauropsida (Gr. aavpos, lizard, and oyjris, appearance), including 

 the reptiles and the birds; and the Mammalia. The extinct 

 reptiles are extremely interesting ; they were very abundant at 

 one time in the water, on the land, and even in the air, for 

 some were provided with large, membranous wings, much like 

 the bats' (Fig. 318). Some that wandered about on the shore 

 and over the land were veritable giants; the well-preserved 

 skeleton of the Iguanodon (Fig. 319) is nearly ten meters in 

 length, and other fossil reptiles could not have been less than 

 twenty-five to thirty meters long, judging from such of their 

 bones as remain to us. The living Reptilia may be divided into 

 four orders. 



Order 1. Squamata 



The Squamata (Lat. squama, scale) consist of the lizards 

 and snakes, which have often been regarded as distinct orders, 

 but at present they are generally grouped together. They are 

 reptiles having the body covered with horny scales, which 

 develop from the epidermis or outer layer of the skin, ami 

 often bony plates from the derma or deeper layer of the 

 integument as well. The outer horny layer of the skin is cast 

 off periodically in all the Squamata, - in most of the serpents 



