BRANCH CHORD AT A: CLASS REPTILIA 3" 



stretched very wide so as to permit the swallowing of 

 large animals whole. Reptiles breathe solely by lungs, 

 of which there is a pair They are simple and sac-like, 

 the left lung being often much smaller than the other. 

 In turtles and crocodiles the lungs are divided internally 

 by septa into a number of chambers. Because of the 

 rigidity of the carapace or " box " of turtles the air cannot 

 be taken in the ordinary way by the use of the ribs and 

 rib-muscles, but has to be swallowed. The reptilian heart 

 consists of two distinct auricles and of two ventricles, 

 which in most reptiles are only incompletely divided, the 

 division into right and left ventricles being complete only 

 among the crocodiles and alligators, the most highly 

 organized of living reptiles. 



The organs of the nervous system reach a considerable 

 degree of development in the animals of this class. The 

 brain in size and complexity is plainly superior to the 

 batrachian brain and resembles quite closely that of birds. 

 Of the organs of special sense those of touch are limited 

 to special papillae in the skin of certain snakes and many 

 lizards. Taste seems to be little developed, but olfactory 

 organs of considerable complexity are present in most 

 forms, and consist of a pair of nostrils with olfactory 

 papillae on their inner surfaces. The ears vary much in 

 degree of organization, crocodiles and alligators being the 

 only reptiles with a well-defined outer ear. This consists 

 of a dermal flap covering a tympanum. Eyes are always 

 present and are highly developed. They resemble the 

 eyes of birds in many particulars. All reptiles, excepting 

 the snakes and a few lizards, have movable eyelids, in- 

 cluding a nictitating membrane like that of the birds. 

 With the snakes the eye is protected by the outer skin, 

 which remains intact over it, but is transparent and 

 thickened to form a lens just over the inner eye. Turtles 

 and lizards have a ring of bony plates surrounding the 



