550 ICHTHYOLOGY. 



the mouth crowded with almost numberless teeth, the Carp and 

 the Roach, Cyprinida, have only a few strong ones in the throat 

 and a single flat one above ; and the Sturgeon, the Pipe-fish, 

 Syngnathus, and the Sand-launce, Ammocates, (Gr. sand-bedded,) 

 are entirely toothless. 



The skin is either naked or covered with scales which appear 

 in various .states of development, as true imbricated scales, as 

 isolated scales, as spiny bristles, hard, bony, enamelled plates. 

 Most of the fishes have on their sides a longitudinal row of scales, 

 in each of which is a perforation. These perforations were for- 

 merly thought to secrete mucus ; but Prof. Agassiz has proved 

 them to be the openings of tubes, which together with similar 

 tubes opening on the skull, penetrate all parts of the frame and 

 freely admit water, which serves to counterbalance the external 

 pressure. 



The colors of Fishes, including as they do all shades and lus- 

 tres, are not surpassed even by those of the Birds. The hues 

 are, however, evanescent, disappearing immediately after death. 

 The effect of fear in changing the color of the human hair is well 

 known. From the statement of a writer in the "New Sporting 

 Magazine," it would appear to have a similar effect upon some 

 Fishes, particularly Trout. 



The brain is small and the face has not much expression ; the 

 tongue is mostly cartilaginous, and sometimes covered with teeth. 

 This, connected with the fact that the food is almost always swal- 

 lowed whole as soon as it is seized, seems to warrant the infer- 

 ence that Fishes have not acute taste. The sense of smell they 

 probably possess in considerable perfection, the olfactory nerves 

 being very large, and distributed over a great extent of surface. 

 There is no external ear, nor even an auditory orifice, yet there 

 is a complex internal apparatus of large size for the reception 

 of sounds. 



The eyes are distinguished by their almost immovable posi- 

 tion ; the cornea is flat, but the crystalline lens is perfectly spher- 

 ical ; the latter is familiar in the form of a white globule in a 

 boiled fish, the transparency being destroyed by heat ; eye-lids 

 are not present. From the density of the watery medium inhab- 

 ited by Fishes, a large number of the rays of light are absorbed 

 and lost in passing through it ; hence the eyes of fishes are very 

 large, so as to collect as many of the remaining rays as possible ; 

 they are also of brilliant hues. These hues are owing to a 

 membrane called the choroid, spread around the back of the eye, 

 composed, to a large extent, of highly reflecting microscopic 

 crystals. The eyes of some species gleam like quadrupeds. 



