The fishes form the last class of vertebrate animals. Their most marked feature of distinction from the other vertebrates is their special adaptation for the element in which they live, and their consequent unfitness to live out of it.
This might be also said with a large amount of truth of the whale family of mammals. They are all formed for a water life, and would soon die if they were left stranded on the dry land. But they would die only through their inability to obtain food, and from no other cause whatever. They would be starved out of the water. They live in the water because their food is in the water, but they breathe through lungs, and must come to the surface of the waves to do it. The fish, taken out of the water, dies from its inability to breathe in the atmosphere. It is suffocated in the new element, as the whale would be if it were immersed for any considerable time beneath the waves.
The amount of oxygen taken in by their gill-breathing is not sufficient to warm the blood, consequently fishes are all cold-blooded animals. The heart has only two chambers-an auricle and a ventricle.
Most fishes possess a curious air-bladder, which enables them, at will, to descend to the bottom, or rise to the surface of the water. The bladder itself is provided with strong muscles. When these muscles contract, the bag is made smaller, the air in it is compressed, and the fishbecomes heavier than the water, so that it sinks with ease. In order to rise to the surface again the fish has only to swell out the bag to its original size. This makes the body lighter, and it ascends as easily as it sank. Without this wonderful piece of mechanism, such movements would be impossible. Certain fish, such as the turbot,sole, and plaice, invariably live at or near the bottom of the water, and never move far from it. These have no air- bladder. It is not required. Fishes are classified according to the nature of the skeleton. In most fishes the skeletonis made of a hard bony substance, as is the case with the other vertebrate animals. These are called i n s c i e n t i f i c l a n g u a g e osseous fishes. The word osseous means bony.
Others have no actual bones, but only gristle or cartilage to form their skeleton. These are known as cartilaginous fishes. The order is a very small o n e , a n d i n c l u d e s t h e shark, dog-fish, aw-fish,skate, and sturgeon.
The osseous fishes are placed in two groups, acording to the nature of their fins. If you examine a fresh herring, sole, plaice, turbot, druon, or eel, you will find that all of them have soft flexible fins. We call them soft-finned fishes. You would have no difficulty in realising the distinction if you placed side by side with them a mackerel, a perch, or a little stickleback. The fins of these are anything but soft and flexible. They are sharp, bony spines. These spines give the name to the fishes of this group. We call them spine-finned fishes. They form a much smaller group than the soft-finned fishes.
Lesson 48