454 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



its head half in and half out of the water, the cornea 

 is divided into two halves by a horizontal line of 

 conjimctival epithelium, and the pupil is similarly 

 divided into an upper and a lower half. The eye is 

 kept in shape and position by the cartilaginous or 

 fibrous condition of the sclerotic ; in bony fishes plates 

 of bone are not unfrequently deposited in this enve- 

 lope or coat of the eye. 



The eyes of the Urodela and Csecilire are small, 

 and this is especially the case in the latter group, 

 the members of which are of burrowing habits ; in 

 both, the skin is completely continued over the eye, 

 and in the Csecilise it is often quite thick. In Proteus, 

 the skin which covers the lens is not at all trans- 

 parent, and this cave-dwelling animal is so far blind 

 that it is apparently only able to distinguish between 

 light and darkness ; in much the same way, probably, as 

 when a man turns his face to the light and closes his 

 eyes, he is still able to perceive the passage of an 

 opaque object, such as a hand, between himself and 

 the light. Just as the cornea of fishes is flattened, so 

 that of the amphibious newt and of the frog is pro- 

 vided with a muscle by which the eye-ball can be re- 

 tracted when the animal is in the water. 



In all Reptiles the eyes are small ; but, partly owing 

 to the length of their bodies, we are especially struck 

 with the smallness of the eyes of Snakes ; in them the 

 pupil is generally rounded, but in some nocturnal 

 species it has the form of a vertical slit. There are 

 no eyelids, or, in other words, the skin is continued 

 over the eye, and this part of the integument is shed 

 with the rest of the skin. In most other Reptiles 

 there are two eyelids, in addition to the "nictitating 

 membrane " which is found in some sharks and Am- 

 phibia, as well as in Birds, and which is drawn over 

 the eye by special muscles. The eye of the crocodile 

 is small. 



