Fish Of Tropical Africa

Fish are animals which live entirely in water. Their shape, which is long and tapering both at head and tail, is best suited for swimming, and so is their body-covering of slippery scales. Instead of four legs for walking they have four fins for swimming, arranged two in front and two behind and working as paddles. The tail is also a large fin and acts as a rudder to guide the fish and as a strong extra paddle. In addition, there are usually one or two fins along the middle of the back and a small single one underneath. Fish feed on smaller living things in the water, and have a stomach storehouse and a blood circulation to carry the nourishment to all parts of the body. But the blood of fish is cold and not warm as in mammals and birds. Water contains a great deal of air dissolved in it, just as sugar will dissolve. When water is heated the dissolved air comes off it as bubbles. It is this air that fishes have to breathe, and for this they are provided with special arrangements for breathing called "gills"; these do the same work that lungs do in other animals. Gills are large slits in the fishes' necks with feather-like folds of skin projecting into each slit. These feathery folds of skin are full of blood-vessel. The water passes into the mouth and out through the gill slits, and the air in the water is taken up by the blood-vessels in the gills. The fish air purifies the blood and the impure air is carried away by the water flowing through the gills. In most fishes there are four pairs of gills, but in the sharks there are five or six, and the same in the mud-fish. In the matter of breathing the mud-fishes are very interesting, As they come to the surface to breathe and are provided with small lungs as well as with gills.