adopt to accomplish this, is to perforate the sound, or air-bladder, with a needle, which disengages the air, when the fishes immediately descend to the bottom of the well, into which they are thrown. Without this operation, it would be impossible to keep the cod under water whilst they had life. In swimming, the fins enable fishes to preserve their upright position, especially those of the belly, which act like two feet. Without those, they would swim with their bellies upward, as it is in their backs that the centre of gravity lies. In ascending and descending, these are likewise of great assistance, as they contract and expand accordingly. The tail is an instrument of great muscular force, and largely assists the fish in all its motions. In some instances it acts like the rudder of a ship, and enables it to turn sideways; and when moved from side to side with a quick vibratory motion, fishes are made, in the same manner as the "screw" propeller makes a steamship, to dart forward with a celerity proportioned to the muscular force with which it is employed.
202. THE BODIES OF FISHES are mostly covered with a kind of horny scales; but some are almost entirely without them, or have them so minute as to be almost invisible; as is the case with the eel. The object of these is to preserve them from injury by the pressure of the water, or the sudden contact with pebbles, rocks, or sea-weeds. Others, again, are enveloped in a fatty, oleaginous substance, also intended as a defence against the friction of the water; and those in which the scales are small, are supplied with a larger quantity of slimy matter.
203. THE RESPIRATION OF FISHES is effected by means of those comb-like organs which are placed on each side of the neck, and which are called gills. It is curious to watch the process of breathing as it is performed by the finny tribes. It seems to be so continuous, that it might almost pass for an illustration of the vexed problem which conceals the secret of perpetual motion. In performing it, they fill their mouths with water, which they drive backwards with a force so great as to open the large flap, to allow it to escape behind. In this operation all, or a great portion, of the air contained in the water, is left among the feather-like processes of the gills, and is carried into the body, there to perform its part in the animal economy. In proof of this, it has been ascertained that, if the water in which fishes are put, is, by any means, denuded of its air, they immediately seek the surface, and begin to gasp for it. Hence, distilled water is to them what a vacuum made by an air-pump, is to most other animals. For this reason, when a fishpond, or other aqueous receptacle in which fishes are kept, is entirely frozen over, it is necessary to make holes in the ice, not so especially for the purpose of feeding them, as for that of giving them air to breathe.
204. THE POSITIONS OF THE TEETH OF FISHES are well calculated to excite our amazement; for, in some cases, these are situated in the jaws, sometimes on the tongue or palate, and sometimes even in the throat. They are in general sharp-pointed and immovable; but in the carp they are obtuse, and in the pike so easily moved as to seem to have no deeper hold than such as the mere skin can afford. In the herring, the tongue is set with teeth, to enable it the better, it is supposed, to retain its food.
205. ALTHOUGH NATURALISTS HAVE DIVIDED FISHES into two great tribes, the osseous and the cartilaginous, yet the distinction is not very precise; for the first have a great deal of cartilage, and the second, at any rate, a portion of calcareous matter in their bones. It may, therefore, be said that the bones of fishes form a kind of intermediate substance between true bones and cartilages. The backbone extends through the whole length of the body, and consists of vertebrae, strong and thick towards the head, but weaker and more slender as it approaches the tail. Each species has a determinate number of vertebrae, which are increased in size in proportion with the body. The ribs are attached to the processes of the vertebrae, and inclose the breast and abdomen. Some kinds, as the rays, have no ribs; whilst others, as the sturgeon and eel, have very short ones. 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 scapulae or shoulder-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 other smaller ones, placed between the muscles to assist their motion.
206. SOME OF THE ORGANS OF SENSE IN FISHES are supposed to be possessed by them in a high degree, and others much more imperfectly. Of the latter kind are the senses of touch and taste, which are believed to be very slightly developed. On the other hand, those of hearing, seeing, and smelling, are ascertained to be acute, but the first in a lesser degree than both the second and third. Their possession of an auditory organ was long doubted, and even denied by some physiologists; but it has been found placed on the sides of the skull, or in the cavity which contains the brain. It occupies a position entirely distinct and detached from the skull, and, in this respect, differs in the local disposition of the same sense in birds and quadrupeds. In some fishes, as in those of the ray kind, the organ is wholly encompassed by those parts which contain the cavity of the skull; whilst in the cod and salmon kind it is in the part within the skull. Its structure is, in every way, much more simple than that of the same sense in those animals which live entirely in the air; but there is no doubt that they have the adaptation suitable to their condition. In some genera, as in the rays, the external orifice or ear is very small, and is placed in the upper surface of the head; whilst in others there is no visible external orifice whatever. However perfect the sight of fishes may be, experience has shown that this sense is of much less use to them than that of smelling, in searching for their food. The optic nerves in fishes have this peculiarity—that they are not confounded with one another in their middle progress between their origin and their orbit. 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 that which comes from the right goes distinctly to the left. In the greater part of them, the eye is covered with the same transparent skin that covers the rest of the head. The object of this arrangement, perhaps, is to defend it from the action of the water, as there are no eyelids. The globe in front is somewhat depressed, and is furnished behind with a muscle, which serves to lengthen or flatten it, according to the necessities of the animal. The crystalline humour, which in quadrupeds is flattened, is, in fishes, nearly globular. The organ of smelling in fishes is large, and is endued, at its entry, with a dilating and contracting power, which is employed as the wants of the animal may require. It is mostly by the acuteness of their smell that fishes are enabled to discover their food; for their tongue is not designed for nice sensation, being of too firm a cartilaginous substance for this purpose.
207. WITH RESPECT TO THE FOOD OF FISHES, this is almost universally found in their own element. They are mostly carnivorous, though they seize upon almost anything that comes in their way: they even devour their own offspring, and manifest a particular predilection for all living creatures. Those, to which Nature has meted out mouths of the greatest capacity, would seem to pursue everything with life, and frequently engage in fierce conflicts with their prey. The animal with the largest mouth is usually the victor; and he has no sooner conquered his foe than he devours him. Innumerable shoals of one species pursue those of another, with a ferocity which draws them from the pole to the equator, through all the varying temperatures and depths of their boundless domain. In these pursuits a scene of universal violence is the result; and many species must have become extinct, had not Nature accurately proportioned the means of escape, the production, and the numbers, to the extent and variety of the danger to which they are exposed. Hence the smaller species are not only more numerous, but more productive than the larger; whilst their instinct leads them in search of food and safety near the shores, where, from the shallowness of the waters, many of their foes are unable to follow them.
208. THE FECUNDITY OF FISHES has been the wonder of every natural philosopher whose attention has been attracted to the subject. They are in general oviparous, or egg-producing; but there are a few, such as the eel and the blenny, which are viviparous, or produce their young alive. The males have the milt and the females the roe; but some individuals, as the sturgeon and the cod tribes, are said to contain both. The greater number deposit their spawn in the sand or gravel; but some of those which dwell in the depths of the ocean attach their eggs to sea-weeds. In every instance, however, their fruitfulness far surpasses that of any other race of animals. According to Lewenhoeck, the cod annually spawns upwards