" The optic nerves in fishes are not con founded with one another in their mid dle progress between their origin and the orbit, but the one passes over the other without any communication ; so that the nerve which comes from the left side of the brain goes distinctly to the right eye, and vice versa. Indeed it should seem not to be necessary for the optic nerves of fishes to have the same kind of connection with each other as those of man have ; for their eyes are not placed in the fore-part, but in the sides of the head ; and, consequently, cannot look so conveniently at any object with both eyes at the same time. The crystalline lens in fishes is a complete sphere, and more dense than in terrestrial animals, that the rays of light coming from the water might be sufficiently refracted. As fishes are continually exposed to injuries in the uncertain element in which they reside, and as they are in perpetual dan ger of becoming a prey to the larger ones, it was necessary that their eyes should never be shut ; and as the cornea is sufficiently washed by the element they live in, they are not provided with palpebrze ; but, as in the current itself the eye must be exposed to several injuries, there was a necessity that it should be sufficiently defended ; which, in effect, it is, by a firm pellucid membrane, seem ing to be a continuation of the cuticula stretched over it : the epidermis is very proper for this purpose, as being insen sible, and destitute of vessels, and conse not liable to obstructions, and thus becoming opaque. In the eye of the skate tribe there is a digitated cur thin, which hangs over the pupil, and which may shut out the light when the animal rests, being somewhst similar to the tunics adnata of other animals." We now-proceed to notice the motion of fishes, for the celerity of which their shape is admirably adapted : hence, ves sels designed to be navigated in water are made to imitate, in some degree or other, the shape of fish ; but the rapidity of a ship in sailing before the wind is not to be compared to the velocity of a fish. The largest fishes are known to overtake a ship in full sail with the greatest ease, to play round it without effort, and to surpass it at pleasure. Every part of the body seems formed for dispatch : the fins, the tail, and the motion of the whole back-bone assist in the business; and it is to that flexibility of body which mocks the effort of art, that fishes owe the great velocity of their motions. The chief in struments in a fish's motion are its fins, air-bladder and tail ; with two pair, and three single fins, it will migrate a thou sand leagues in a season, and without in dtcating any visible symptoms of languor or fatigue. The fins serve not only to assist the animal in progression, but in rising and sinking, in turning, and even in leaping out of the water. The pecto ral fins serve to push the animal forward, and to balance the head when it is too large for the body, and prevent it from tumbling to the bottom, which it infalli bly would if the fins were cut off. The ventral fins, which always lie flat in the water, serve rather to raise or depress. the body, than to assist its progressive motion. The dorsal fin acts as a poiser, in preserving the animal's equilibrium, while it aids the forward movement ; and the anal fin is designed to maintain the vertical position of the body. By means of the air bladder, fishes can increase or diminish the specific gravity of their body. When they contract it, and press out the air, the bulk of the body is diminished, and the fish sinks as far as it pleases : on relaxing the operation, the bladder ac quires its natural size, the body becomes specifically lighter, and the fish is ena bled to swim near the surface. The tail, in the last place, may be regarded as the rudder, directing the motions of the fish, to which the fins are only subser vient.
With respect to the nourishment of fishes : they are mostly carnivorous, though they seize upon almost any thing that falls in their way, and not uncommon ly devour their own offspring : they seem, indeed, to manifest a particular predilection for whatever they can swal low possessed of life. They often meet with
each other in fierce opposition, and the victor, without scruple, devours his anta gonist. Thus are they irritated by the continual desire of satisfying their hun ger ; and the life of a fish, from the small est to the greatest, is but one scene of hostility and violence. The smaller spe cies, which stand no chance in the un equal combat, resort to those shallows where the larger are unable to approach. There they become invaders in their turn, and live on the spawn of large fishes, and on small insects and worms, which they find floating on the water. Fishes can, however, notwithstanding their na tural voracity, live long, apparently, with out food ; but they, perhaps, in vases and other ornamental vessels, feed on insects too small for the human eye to see ; or, it has been thought, they may have the power of chemically decomposing water. We now proceed to the subject of repro duction.
In most, if not in all fishes, there is a dif ference in sex, though Bloch and others make mention of individuals, which seem ed to unite the two sexes, and to be real hermaphrodites. The number of males, it has been remarked, is about double that of females ; and were it not for this wise provision of nature, a large propor tion of the extruded eggs would remain unfecundated. A few species, indeed, as the eel, blenny, &c. are viviparous ; but by far the greater number are produced from eggs. These last compose the roe, ovaries of the females, which lie along within the abdomen. The milt of the males is disposed along the back-bone, in one or two bags, and consists of a whitish glandular substance, which secretes the spermatic fluid. Though the history of the generation of fishes be still involved in considerable obscurity, it seems to be ascertained, that no sexual union takes place among the oviparous kinds, and that the eggs are fructified after each', sion. They are of a spherical form, and consist of a yolk, a white part, and a bright crescent-like spot, or germ. The yolk, which is usually surrounded by the white, is round, and not placed in the middle, but towards one of the sides ; and the clear spot, or embryo, is situated be tween the yolk and the white.
In this spot there is observable, on the day after fecundation, a moveable point, of a somewhat dull appearance. On the third day, it assumes the appearance of a thickish mass, detached on one side, and on the other strongly adhering to the yolk, and presenting the contour of the heart, which at this period receives an increase of motion, while the disengaged extremity, which forms the rudiments of the tail, is perceived to move at intervals. On the fourth day, the pulsations of the heart, and the movements of the whole body occur in quicker succession. On the fifth, the circulation of the hum2urs in the ves sels may be discerned, when the fish is in a particular position. On the sixth, the back-bone may be distinctly recognised. On the seventh, two black points, which are the eyes, and the whole form of the animal, are visible to the naked eye. Al though the yolk gradually diminishes as the embryo enlarges, the included animal cannot yet stretch itself at length, but makes a curve with its tail. Its motions are then so brisk, that when it turns its body, the yolk turns with it; and these motions become more and more frequent, as the moment of birth, which happens between the seventh and ninth day, ap proaches. By repeated strokes of the tail, the covering of the egg at length gives way, and the fish comes forth, first by the tail, redoubling its efforts, till it detach its head ; and then it moves nimbly, and at liberty in its new element.