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 in 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 as 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. It is in 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- 



