REPTILES 209 



The breathing organs and parts connected with them differ 

 from those of the lizard not only in the arrangement of the 

 air-passages as already described, but also in the structure of 

 the lungs themselves, which have relatively thick spongy walls. 

 A similar arrangement, however, is found in some large lizards, 

 and, speaking generally, it is only small reptiles which have very 

 simple lungs. Crocodiles also possess an imperfectly-developed 

 representative of the muscular partition known as midriff or 

 diaphragm, which plays such an important part in the breathing 

 movements of Mammals (see p. 46). 



The brain of a crocodile is of higher type than a lizard's, 

 both cerebral hemispheres and cerebellum being much larger 

 in proportion. The sense organs are also more complex, espe- 

 cially those connected with smell and hearing. In the latter 

 case it is interesting to note that the complex bag known as the 

 membranous labyrinth (fig. 103), which constitutes the essential 

 part of the inner ear, is shaped very much as in a bird, there 

 being a well-developed curved tube to represent the coiled cochlea 

 of Mammals. 



Crocodiles are hatched out from eggs much resembling those 

 of birds, and laid in holes or imperfect nests scooped out in the 

 sand, where they get the benefit of the sun's heat. The young 

 reptile has a horny wart on the tip of its snout, which helps it 

 to break through the firm egg-shell. 



The existing members of ' the order Crocodilia (fig. 133) 

 are found in tropical and sub-tropical regions both in the New 

 and Old Worlds. Three groups are commonly recognized: — 

 (i) Alligators; (2) Crocodiles proper; and (3) Garials. 



Alligators, with the exception of a Chinese species, are 

 confined to the tropical parts of America. The snout is 

 relatively short and broad, and the different teeth are of very 

 unequal size, among the large ones being the first and fourth 

 in the lower jaw, which bite into pits in the upper jaw in 

 such a way that they are hidden from view when the mouth is 

 shut. There are several special features in the skull, of which 

 perhaps the most striking is the shortness of the region where 

 the two halves of the lower jaw unite together in front. Some 

 members of the Alligator group possess scutes on the under 

 side of the body as well as on the upper. The hind-limbs 

 have a rounded outline and the toes are only half-webbed. 



vol. I. 14 



