210 NATURAL HISTORY. 
the blood of Mammals, Birds, and Reptiles does in the 
lungs. But, while their blood returns to the heart from 
the lungs before it is distributed over the system, the 
blood of the Fish is distributed directly from the gills. 
352. The circulation of Fishes is not as active as that 
of Mammals and Birds, and their blood is cold like that 
of Reptiles. We can readily see why it is best that it 
should be cold. There are only two ways in which it 
could be kept warm, like that of warm-blooded animals. 
One is by having a covering of feathers or of fur, as in 
the case of animals living im air. But such a covering 
would interfere very much with swimming, Another 
way to retain the heat would be to have, like the whales, 
a thick layer of fat under the skin. This would be very 
burdensome ; and, besides, man does not need such a sup- 
ply of fat and oil as this arrangement in the Fishes would 
give him. : 
353. The shape of the Fish is such 4s to let it move eas- 
ily through the water. It has, commonly, a long, spin- 
dle-like shape, with an even surface. It has no neck, 
chiefly because any irregularity in its surface would hin- 
der its rapidity of motion. Its outer covering favors its 
gliding through the water, for it is generally composed 
of smooth scales, one overlapping another, like shingles 
or tiles. Then there is a slimy, oily secretion over the 
whole surface, helping it to move smoothly through the 
water. 
354. The Fish is nearly of the same specific gravity with 
water. It is therefore obliged to make very little effort 
in going upward. Itisin strong contrast with the Birds 
in this respect. A bird, in mounting upward, exerts 
great force with its broad wings and its large muscles, 
because it is in an element which is so much lighter than 
itself. But as the fish is in an element only a little light- 
er than itself, it needs but a small apparatus to move in 
it, and, accordingly, its tail and fins are much smaller in 
proportion to its bulk than are the wings of birds in pro- 
