44 
FIRST BOOK. 
THE SHARK. 
1. The sea is the home of very many kinds of 
fishes. Though these differ very much in size 
and in shape, they are alike in being able to 
breathe and live only in water. 
2. Most likely you have often seen a fish. Did 
you see the scales on its skin? If the fish had 
fur or feathers it could not pass so easily through 
the water. And did you notice the gills at each 
side of its neck, and 
the fins which it used 
when swimming? 
3. Fishes breathe by 
their gills. They take 
water in at the mouth, 
and force it out through 
the gill-slits, or holes, in 
the neck. On its way 
out it runs over the gills, where numbers of fine 
blood-vessels are spread out. so that the air in 
the water can get to them. 
4. That is the way the fish's blood is kept pure 
and healthy, just as ours is kept pure by the air 
which we breathe into our lungs. 
5. In most fishes the gills are in flat layers, and 
divided like the teeth of a comb. They are also 
hidden by a stiff covering, which acts like a lid, 
