SENSE-ORGANS 139 



therefore intended more for concealing or protecting 

 than for feeling. 



The Hearing of reptiles is quite acute. Though 

 they have much better ears than any creature below 

 them these are far from being so complete as those of 

 the mammi^ls or even those of the birds. The ears are 

 connected with the mouth by tubes — in the croco- 

 dilians by three openings. This forms the internal 

 ear. Besides this there is a middle ear of more or 

 less incompleteness. Except in crocodiles, where 

 there is an external fold or flap of skin, there is no 

 outside ear. Here there are some valves over an 

 opening to keep the water out, but in many reptiles 

 the drum-membrane lies on the outside of the head, 

 as in the frogs, except that it may be protected by 

 special scales. In others, as the serpents and Tua- 

 tera, there is no drum-cavity ; and in the latter only, 

 it is said, are there any signs of that wonderfully 

 spiral arrangement called the cochlea, which allows 

 us to appreciate the difference between high and low 

 tones in pitch. 



All reptiles see, there being none known that 

 are perfectly blind, but some burrowing forms are 

 nearly so. Sight in this class, however, has no such 

 keenness as in the birds. Reptiles usually have two 

 eyelids, and some have that third kind called a nicti- 

 tating membrane. The skinks have a transparent 

 window in the lower lid, so that while burrowing 

 slightly in deserts they may see out without getting 

 sand in the eye. Tear-glands here prevail for the 

 first time in vertebrates. As noted, snakes and some 



