THE whole of creation has been divided by scientists into six sub-divisions of which one contains the Vertebrates—that is, all those animals which have backbones. These creatures are again divided into five great classes according to whether they are fish, amphibians, reptiles, birds or mammals.
The term vertebrate may be taken to include all animals which have a symmetrical form on both sides. The body is divided by an imaginary axis into a right and left half, and the nerves and muscles in each are correspondingly formed and placed. The main masses of the nervous system lie along the back, and are always shut off from the internal organs. The nerve axis is underlaid by a structure known as the ‘notochord,’ which is a supporting rod, and in the adult stage is generally more or less replaced by the bony axis known as the ‘vertebral column.’ There may be no limbs, but when present there are never more than two pair of them.
Fish form the largest class in point of numbers and are among the most ancient of vertebrates, whilst its members exhibit the greatest variety of structure. All fish have gills throughout their life; the structure of the heart, when present, is usually simple; the limbs are in the form of fins. The shape of the body is largely based on its suitability for rapid movement in water, and for the avoidance of friction. To help further in this direction, an outer layer of mucus, or slime, covers the skin.
The structure of the scales is of four kinds. The scales may be, first: thin, bony and flexible (as in the fish, and the most familiar form to the majority of us); secondly : thin, bony, horny plates with spiny or comb-like projections; thirdly: scales covered with a hard, polished enamel; or, fourthly : detached plates or tubercles, which are often covered with spines.
Fish are essentially inhabitants of water, either fresh or salt, though a few live in both impartially. Fish like eels can live a considerable time out of water owing to the narrowness of their gill slits or because they have a special arrangement for keeping the gills moist.
With notable exceptions, some of which are mentioned below, fish reproduce by external fertilisation. The female discharges her eggs into the water near the male, who, about the same time, discharges sperm, also into the water. In this way some of the eggs are fertilized and, unless destroyed by other creatures, eventually hatch out into young fish.