4 THE FISH. 



Besides the common parts of the brain, which are 

 placed as in reptiles behind each other, the fish have 

 also tubercles at the base of the olfactory nerves. 



Their nostrils are simple fossae at the end of the 

 muzzle, almost always pierced with two holes, and 

 lined with a pituitary membrane regularly plaited. 



The eye has its cornea very flat, the aqueous 

 humour little, but the crystalline nearly globular, and 

 very hard. 



The ear consists of a sac, representing the vestibule, 

 and containing in suspension small masses, generally 

 of a stony hardness, and moreover of three semicir- 

 cular membranaceous canals, rather situated in the 

 cavity of the cranium than engaged in the substance 

 of its own parietes, except in the chondropterygians ; 

 there are neither eustachian tube nor tympanal bones, 

 and the selacians alone have an oval opening (fenestra 

 ovalis), but on the level of the head. 



Taste in fish cannot be very acute, since their 

 tongue is in great part bony, and is often furnished 

 with teeth or other hard coverings. 



The majority, as every one knows, have the body 

 covered with scales ; all are destitute of any organ 

 for holding ; certain little fleshy tendrils which some 

 of them possess may supply the imperfection of touch 

 in the other organs. 



The intermaxillary bone forms in most of them the 

 edge of the upper jaw, and behind it the maxillary, 

 commonly called os labiale ; a palatine arch, composed 

 of the palatine, the two pterygoidian processes, a 



