gill-chambers by an aperture, the "gill-slit," placed at sides of the mouth behind the "gill-cover," an organ consisting of a chain of flat bones and a membrane. The oxygen, after passing through the gills, is driven through all parts of the body, and the venous and impure blood is forced by the action of the heart to the gills, where it is subjected to the action of the water. Oxygen is essential to the life of a fish, and suffocation results unless that gas is present in water. The teeth of fish are in the jaws, sometimes on the palate or tongue, and in some cases they are placed in the throat. They are usually sharp-pointed and fixed; in the carp they are obtuse, and in the pike they are easily moved. When lost or injured, the teeth of fish are replaced; they are not set in sockets, but are attached by a ligament to the bones of the mouth. In the herring the tongue is set with teeth, by means of which it more easily retains its food.
These two great Divisions formed the basis of the classification of the eminent naturalist, Cuvier, and may serve roughly to differentiate the various classes of fish which are now more scientifically grouped according to their particular characteristics into six divisions, or four orders, if the classification of Agassiz, based on the structure of their scales, be followed. The true internal skeleton differs very widely among fish; in the case of one fish, the lancelet, a true skeleton scarcely exists, the backbone being replaced by a soft cellular rod. In some, for example, the lampreys, sturgeons and rays, it is cartilaginous; in others it is partly cartilaginous and partly bony; and in a great many, like the herring, perch, etc., it is entirely composed of bone. The backbone extends through the whole length of the body, and consists of vertebrae, strong and thick towards the head, but weaker towards the tail. Each species has a determinate number of vertebrae, which are increased in size in proportion to the body. The ribs are attached to the processes of the vertebrae, and enclose the breast and abdomen. Some fish, the rays, for instance, have no ribs; whilst others, like the sturgeon and eel, have them very short. Between the pointed processes of the vertebrae are situated the bones which support the dorsal (back) and the anal (below the tail) fins, which are connected with the processes by a ligament. At the breast are the sternum, or breastbone, clavicles, or collar-bones, and the scapullae, or shouder-blades, on which the pectoral or breast fins are placed. The bones which support the ventral or belly fins are called the ossa pelvis. Besides these principal bones, there are often smaller ones, placed between the muscles, which assist their motion.
The Organs of Sense.—The organs of sight, hearing, smelling, taste and touch are possessed in a higher or lower degree by fish. Those of taste and touch are the least developed. The filaments at the mouths of the cod, sturgeon and whiting are supposed to be organs of touch, and it is also thought that the "lateral line," running along the sides