br, the branchiæ, or gill openings; c, the heart;
f, the liver; vn, swimming bladders;
ci, intestinal canal; o, the ovarium;
u, urethra; a, anus; o', oviduct.
Immediately behind the head, two large openings are observed in most fishes; these are the gill-openings. Their anterior edge is mobile, and they are raised or lowered to serve the purposes of respiration; under this species of covercle are the gills, or branchiæ. These usually consist of many rows of thin membranous plates, hung on slender arches of bone, placed on each side of the head, usually protected by a bony plate made up of several pieces, called the gill-covers. The breathing is produced by water taken in at the mouth, which passes over the gill-membranes, and is ejected through an orifice at the hind margin of the gill-covers. During the contact of the water with the gills, the blood which circulates in these organs, and which communicates to them the red colour by which we recognize them, combines chemically with the oxygen of the air which the water holds in solution when it flows freely at the ordinary temperature in presence of the air. The blood is thus oxygenized, or made fit for respiration.
The heart in fishes is placed between the inferior parts of the branchial arch, and consists of a ventricle and an auricle (Fig. 342). It corresponds with the right half of the heart in the Mammifera and birds, for it receives the venous blood from all parts of the body and sends it to the gills. From this organ the blood is delivered into one great artery, which creeps along the vertebral column.
Fig. 343. A Fish's Eye.
i, crystallized pupil; ee', cornea; mm', choroid; h, posterior chambers; c, optic nerve.
The eye in fishes is generally very large—we may even say enormous relative to the size of the head—and without true eyelids; the skin usually passes over the ocular globe, and becomes from this point so transparent that the luminary rays traverse it. This light covering is all the eyelid belonging to fishes. The interior of the eye is covered by the membrane called choroid, the thin external leaf of which, in consequence of the presence of innumerable microscopic crystals, presents the appearance of a gold or silver-coloured coating, which gives to the iris that extraordinary brilliancy which belongs to the fish's eye. The crystalline lens is voluminous, spherical, and diaphanous. When the fish is cooked, the crystalline lens constitutes that opaque and hard white substance which often comes under the teeth in eating fish of a certain size. Cuvier suspected, what anglers now know to be true, that those active chasseurs of the deep saw far and very clearly.[16]
Fig. 344. Teeth of the Bream.
If fishes have great eyes, they have, on the other hand, very small ears. This organ, it is found, has no exterior opening. It forms a cavity in the interior of the cranium, which is far from presenting the complicated structure of the ear in mammifers and birds. In spite, however, of the imperfect structure, fishes are sensible to the least noise. In consequence, silence is a rigorous law with the fisherman.