14 



FISHES. 



Fishes, a large number of the rays of light are 

 absorbed and lost in passing through it ; hence 

 their eyes are very large, to collect as many of 

 the remaining rays as possible. The cornea is 

 flat, but the crystalline lens is perfectly spheri- 

 cal ; the latter is familiar in the form of a white 

 globule in a boiled Fish, the transparency being 

 destroyed by heat. The pupil is large, and the 

 iris is almost motionless. Eyelids are not pre- 

 sent; and as the surface of the eye is always 

 bathed by the surrounding 

 water, there is no need for 

 the secretion of tears. 



The eyes of Fishes fre- 

 quently reflect brilliant colours, 

 red, orange, j^ellow, blue, and 

 black, and not unfrequently 

 display a pearly or metallic 

 lustre, like that of e'old or 



EVE OF SWORD-FISH. -i rrj l l 



silver. ihese hues are due 

 to a membrane called the choroid, spread around 

 the back of the eye, composed largely of highly 

 reflecting microscopic crystals. The eyes of some 

 species gleam like those of quadrupeds. 



In general the eyes are placed opposite each 

 other on the two sides of the head, so as to look 

 laterally, and (owing to the tapering of the head) 

 a little forward. In some, however, especially 

 such species as habitually live at the bottom, 

 they are placed on the top of the head, and look 

 vertically upwards. In one genus of Sharks, 

 called, for this reason. Hammer-heads, the head 

 is enormously widened, or lengthened sidewise, 

 so as to present two long lateral processes, at 

 the extremities of which are placed the eyes ; 



