48 STORY OP THE AMPHIBIANS 



be inflated the eyes project more. All the frog- 

 forms have eyelids. The low6r one tends to be trans- 

 parent (like glass), and hence it has been said that 

 they have a third lid or "nictitating membrane." 

 There are no tear glands. Immersed in the water, 

 the amphibian has no need for tears to wet the eye. 

 Many frogs, like some fishes, can roll the balls over 

 in the sockets and thus moisten them. The lids are 

 moist, from other sources. Frogs floating on the water 

 are often seen to immerse the head suddenly and roll 

 the eyes backward as if to wet them. 



In the tailed forms the eyes are much smaller and 

 less perfect. Some have eyelids, but in those which 

 always keep their gills, either external or internal, the 

 eye is usually much like that of some fishes, having 

 no lids, but the outer skin runs directly over them. 

 In Proteus, which lives in a cave in Austria, in Ty- 

 plotriton, found in a cave in Missouri, and in the 

 burrowing csecilians the eyes are covered by the thick 

 skin proper, and th^y remain as mere dots. While 

 the lens is gone, enough of the nerve-matter of the 

 eye remains to enable the creature to tell light from 

 darkness. In one of the csecilians this sort of eye 

 has even sunk beneath the bones of the skull ; but its 

 tadpoles have better eyes, along with a fan tail, and a 

 hint of a leg — all of which show how low these crawl- 

 ing creatures have fallen. 



The flying tree-toad only has large, owl-like eyes, 

 and needs to see at a distance, to make its .tremen- 

 dous leaps. In our slow-going common toad, which 

 is also nocturnal, the eyes are small and dull. 



