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, yellow, blue, and 
black, and not unfrequently 
display a pearly or metallic 
lustre, like that of ’ gold or 
EYE OP SWORD-FISH. -i rril J 
Sliver. 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 ; 
