Breathing

Fish take in water through the mouth; they do not drink it, however, but pass it out backwards through the gill-openings at the sides of the head, under the bony gill-covers. As it goes through it bathes the gills, which are so constructed that they can take oxygen out of the water and get rid of carbon dioxide in exchange. The gills, then, are their breathing organs; they have no lungs. But the process of breathing is otherwise much like ours, and if there is not a proper supply of oxygen in the water, they will sicken and die.

The oxygen is dissolved in the water, and the carbon dioxide given off, through contact with the air at the surface.

Thus it will be understood that if the surface area is too small the water will become charged with carbon dioxide, and there will not be room for replacement of the oxygen when it is used by the fish.

The oxygen that the fish breathes is passed into the blood circulatory system, and so to the tissues, where it is eventually combined with elements of the digested food for the production of energy and growth (metabolism). The results of this combination are largely carbon dioxide and water, which have to be eliminated. The former is breathed out, and the latter excreted by way of the kidneys. This is all much the same as the process in higher animals, including ourselves, for the organization of the body of a fish is very similar to ours, though a little simpler.

Fins

Fish are the lowest true vertebrates, and therefore can be compared with our earliest ancestors. They have a bony skeleton, consisting of a jointed vertebral column or "backbone", with a very complicated skull at the front end; behind the skull there are shoulder-bones supporting a pair of pectoral fins, corresponding to our arms, and beneath the abdomen there is a pair of pelvic fins supported by internal bones, representing our legs.

The latter are sometimes called ventral fins, but this is not a good name, because the word ventral, when applied to fishes, means on the lower side of the body, and can be applied also to the anal fin, which lies below the tail, and is single. On the back, in the middle line of the body, is the dorsal fin, sometimes in two parts. At the end of the tail is the caudal fin; this is often called "the tail" by non-aquarists, but should at the worst be called the "tail-fin".

Senses

A fish is provided with a brain, simpler than ours, but nevertheless more efficient than some people realize; it displays intelligence, and is capable of learning. It has good eyes and a keen sense of smell. The general plan of the nervous system is similar to ours, though again less complex, and it is well to remember that a fish feels pain and may be shocked. Tapping on the glass, or sudden knocks on the frame of the aquarium, should be avoided, for it has a very startling effect on the fishes.

Along the side of a fish is a series of very sensitive organs, which are usually visible as a line formed by tubes in the scales. This is the lateral line, which in effect is the outer ear of the fish. It is sensitive to pressure-waves and vibrations in the surrounding water, and conveys them to the brain. There is an inner ear surprisingly like our own, but concerned mainly with balance, so far as we can tell.

There is of course much more that can be learned about fish, but this information will get you off to a very good start.