CHAPTER II
CHARACTERISTICS AND HABITS OF FISHES
Fishes are the most primitive vertebrate, i.e. backboned, creatures known. All reptiles, birds, and animals have gradually evolved from fish-like ancestors by a series of age-long processes, the stages of which are recorded in fossilized remains that are found in various rock strata throughout the world. A fish lives exclusively in water. It has no lungs, but extracts oxygen from the water as it passes over the surface of its gills. Instead of limbs, it has fins, with which it balances itself and propels itself through the water. Its skin is either bare, e.g. the cat fish, or is covered with scales, e.g. the herring, or with bony plates, e.g. the sturgeon. The skin of certain sharks is studded with minute teeth and produces, when cured, the well-known shagreen leather. In nearly all cases the skin of fishes is liberally supplied with small glands which constantly produce a lubricating mucus. This mucus greatly reduces friction between the fish and the water through which it moves.
The body of a fish is adapted to move swiftly and smoothly through the water; it is shaped more or less like a torpedo, but this form is greatly modified in different species. Certain species of fish living at the bottom of the sea, for example skates and rays, have become flattened, as though by a pressure applied vertically downwards. Others, for example plaice, flounder, sole, appear to have been flattened sideways. In the various members of the eel family, the body is greatly elongated.
Fig. 1
The body of a fish is generally coloured and marked in such a way that it becomes practically invisible when seen from above or below, the under-surface being silvery white, and the upper surface generally olive or blackish-green. Sometimes, as in the mackerel, the upper surface is mottled, resembling rippled water.
Most small fish in ponds and streams reflect their surroundings so well, and are coloured and marked in such a way, that they are almost invisible to the large fish, for example pike, that prey upon them. Generally, they reveal their presence by the flash of light reflected from above by their scales, as they turn suddenly to snap at a morsel of food. In the same way, many predatory fish, e.g. the angler fish, resemble their surroundings so closely that the fish for which they are lying in wait swim within easy reach of them without perceiving their danger. Many fishes, particularly in tropical waters, are remarkable for their bright and gorgeous colouring. It is impossible to preserve these colours in their natural brightness after the fish have been taken from the water, but amongst the brightly coloured corals, and anemones and seaweeds, in the crystal clear water of their natural environment, they flit like gorgeous tropical birds in a tropical forest.