wrinkled skin, clothed not in hair or | feathers, but defended by smooth and glossy scales, lying as flat as possible, and with their free margin directed backwards, all regularly overlapping each other, like the slates of a roof. Besides this, the surface is lubricated by a mucous fluid, which in some, as in the eel, where the skin, from the extreme minuteness of the scales, appears naked, is particularly abundant, and facilitates not only the aquatic progression of the animal, but enables it to creep through small orifices, and insinuate itself into the holes, to which it retires for safety and concealment.

While we allude to the adaptation of these animals for the water, and to their formation for traversing it, with the least possible impediment arising from their own structure, we are called upon to notice an internal organ, respecting which there have been many conjectures, but which most naturalists consider to act the part of a float, enabling them to rise with despatch from great depths, while at the same time it is so under control, as to permit them to re-descend, without causing any obstruction. We allude to that apparatus commonly termed the swimming-bladder, or sound. The swimming-bladder is an elongated membranous sac, variable in form and extent, running along the under surface of the spinal column, and firmly attached to it. It is filled with air, evidently the secretion of a delicate tissue, which lines it internally. In many instances the swimming bladder has no orifice, being closed entirely; but in other instances, as the salmon, sturgeon, herring, carp, etc., the sac communicates with the œsophagus, by means of an orifice, which allows the escape of the air, when the sac is compressed by the action of the abdominal muscles. We have said, that the air contained in this curious sac is secreted by the apparatus itself; and this opinion is demonstrated by various facts. By some naturalists, indeed, it has been imagined, that the air was, in some unaccountable manner, abstracted by the fish from the surrounding water, and transmitted through the gullet into the air bag; but the impossibility of air being thus introduced, when no communication exists between the gullet and this air bag, is in itself at variance with such an hypothesis; and besides, we have yet to learn by what organ or apparatus, either in the mouth or the

gullet, or the stomach of fishes, such abstraction of air from the water is effected. Besides, the air contained in this sac is a gas, which does not enter into the composition of water; and, moreover, it is not atmospheric air, which the water has absorbed, and which it contains mingled with its proper particles. In most instances, it is pure nitrogen; but sometimes this gas is mixed with oxygen, and in some cases the oxygen predominates. Dr. Priestley was the first who turned his attention to this subject, and found that the air bag, or swimming bladder in the roach consisted, in some instances, of azote, or nitrogen, unmixed; in others, of nitrogen, with a small proportion of oxygen. Subsequently, Fourcroy pursued some experiments relative to the gas contained in the air bag, and found it in the carp to consist of almost pure nitrogen. Still more extensive and accurate experiments were undertaken by M. Biot; he found both nitrogen and oxygen in the air bag, but he was unable to detach either carbonic acid or hydrogen: He ascertained, also, that the proportion of oxygen increases, according to the depth at which the fish habitually dwells; though in the sea water itself, whether taken from the surface, or from the greatest depths possible, there is no difference in the proportion of its constituents. More lately, (in 1809,) M. Configliacchi repeated M. Biot's experiments, and confirmed their accuracy. It is then proved, that the air contained in the apparatus in question, inasmuch as it is neither atmospheric air nor hydrogen, but usually nitrogen; and moreover, as it is found to fill the air sac, when this has no aperture, must be the product either of the lining membrane of the sac, or of a red glandular body within, which is sometimes extensive.

In fishes, where the air bag has no orifice or safety valve for the exit of the gas it contains, it is liable, under certain circumstances, to burst; as, for example, when the fish is suddenly drawn up from a considerable depth into the air, and the pressure of the water to which its body had been subjected, instantaneously removed, thereby allowing a rapid and energetic expansion of the previously compressed gas. This effect is frequently produced on codfish, and even in perch and other fishes; and the gas rushing into the general cavity of the abdomen, not only distends it greatly, but even

pushes up the gullet and stomach into the mouth.

A question here suggests itself, Is the swimming bladder an organ which really deserves this appellation, as influencing the fish in locomotion ?-in cleaving the water, either of the river, the lake, or the sea?-in sinking to the bottom, or rising to the surface? There is considerable doubt on the subject. Some physiologists have supposed the apparatus in question to be in some way or other an auxiliary to the gills or the aeration of the blood: and others have advanced a step farther, and regarded it as the representative of the lungs of mammalia, or rather of birds, in which latter the lungs are fixed to the vertebral column, and ribs; but this is a visionary speculation. Most regard it as an apparatus for altering the specific gravity of the fish, and therefore of use in enabling the fish more easily to rise or descend. The compression of this sac, either by the muscles of the abdomen, or of a special muscular apparatus acting upon it, will, of course, force the gas into a smaller space, the specific gravity of the fish increasing in a corresponding ratio: and, on the contrary, the relaxation of these muscles, the pressure being thus removed, will allow the expansion of the gas; and thus the specific gravity of the animal will become lighter: in the former case, its tendency will be to sink, in the latter to rise. Plausible as this theory is, it is not quite satisfactory: in favour of it, however, it may be observed, that flat fish, such as soles, turbot, etc., which reside always at the bottom of the water, are in general destitute of this air vessel; and further, that when in a fish possessing it, it is punctured, and the gas escapes, the animal sinks immediately to the bottom, and is unable, by the exercise of its fins, again to elevate itself. On the other hand, there are many fishes which are remarkable for their rapidity, and for the facility of their movements, in which this apparatus does not exist. The sharks, for example, which plunge down, and rise with extraordinary velocity, and roam at various depths, are destitute of it. The common mackarel, (Scomber scomber,) which is remarkable for activity, and which pursues its prey at the surface, is not furnished with this apparatus, while in a very closely allied species (Scomber pneumatophoris) it is present. But while the swift shark and

the active mackarel are destitute of this air vessel, the conger and the common eel, which habitually reside at the bottom of the water, and burrow in mud, are furnished with it, and it is often extensively developed; perhaps, indeed, but for its presence, the massive conger would be unable to swim, or pursue its prey. On the whole, the evidence is in favour of the generally ascribed use of this air vessel; but, as we have said, the theory is not quite satisfactory, as we cannot explain the reason why some fish, habitually residing at the bottom of the water, should have this organ, and others not; and why, while most fish which roam at various depths possess it, others equally active, and equally capable of descending, or rising, should want it.

It is from the air vessels, or sounds of various fishes, that isinglass is prepared. Those of the codfish and the ling are frequently employed; but those of the sturgeon are in the greatest request, and furnish the best material.

The tenants of an element more dense than our atmosphere, less capable of conveying sound, or of transmitting rays of light, often indeed turbid, from the admixture of extraneous matter, and moreover but little adapted for the diffusion of odorous particles, fishes, as may be anticipated, have not the organs of the senses so delicate, so refined, so discriminating, as have the higher vertebrata. Unlike mammalia and birds, which communicate their instinctive feelings by cries and modulations of voice, these creatures are mute: many, besides, live at a depth where the roaring of the billows, while the surface is tempesttost, cannot reach their realms of silence; to such beings, a keen sense of hearing would be useless. Accordingly, we find the auditory apparatus comparatively rude, (if the term be allowed,) and less developed than in birds and mammalia. The whole organ, indeed, lies deep beneath the skin, and is furnished neither with an external ear, nor a tympanic cavity, and, consequently, neither with a tympanum, (the drum of the ear,) nor with the chain of little bones, which, in the higher vertebrata, communicates between the tympanum and the interior parts of the labyrinth. There is, moreover, no Eustachian tube. The ear of fishes, in fact, consists only of a labyrinth, composed of three semicircular canals, (a a a, in the following sketch,) communicating with a

vestibule, bb, all membranous, and lodged in a wide cavity on each side

of the water in contact with their external surface. That noise produces in fishes a powerful sensation may be admitted; but, as we have said, it is questionable, whether their ear appreciates differences of tone, as it does in birds and mammalia.

of the cranium, and surrounded immediately by a glassy mucilaginous fluid: to each of these canals the auditory nerve c sends a filament, which ultimately spreads in the form of a beautiful network over the inner lining of the vestibule. The vestibule and the canals are filled with a mucilaginous fluid, and the former contains certain calcareous bodies, of an enamel-like texture and extreme hardness, termed otolithes, or earstones, suspended by means of delicate filaments; they are generally three in number; their form differs greatly in the various species, but in every individual of the same species they are themselves the same.

In the cartilaginous fishes, as the shark, etc., these otolithes have neither the enamel gloss, nor the hardness which characterize them in other groups, and they resemble moistened starch, and consist of chalk, with a portion of gela

tine.

There is no cochlea, as in the human ear, into which the vestibule opens; and from this circumstance, and also from the want of a tympanum, it is supposed that these animals cannot distinguish the differences of tone; that all sounds which they do hear, are to them nearly the same, varying only in intensity, according to the more or less violent vibration of the fluid contained in the labyrinth, and the agitation of the otolithes. But the vibration of this fluid must depend, as there is no external orifice, upon the vibrations of the walls of the cavity in which the labyrinth is lodged, and these walls must be affected by the vibrations

The olfactory organs of fishes probably possess a corresponding ratio of sensibility to those of hearing; and it appears to be rather by the sight than the smell that they search for or pursue their prey. We know that they seize artificial flies, and imitations of other fishes, of frogs, and of mice, etc., which, were they guided by the smell, they would not do. We are aware that some physiologists regard the sense of smell in these animals as very acute, and as furnishing them with a most important test, in their acquisition of food. Dr. Munro (Comp. Anat. p. 127, 1783,) says, "If you throw a fresh worm into the water, a fish shall distinguish it at a considerable distance; and that this is not done by the eye is plain, from observing, that after the same worm has been a considerable time in the water, and lost its smell, no fishes will come near it; but if you take out the bait, and make several little incisions into it, so as to let out more of the odoriferous effluvia, it shall have the same effect as formerly. Now it is certain that, had the creatures discovered this bait with their eyes, they would have equally come to it in both cases. In consequence of their smell being the principal means they have of discovering their food, we may frequently observe them allowing themselves to be carried down the stream, that they may ascend leisurely against the current of the water; thus, the odoriferous particles swimming in that medium being applied more forcibly to their organs of smell, produce a stronger sensation."

It is not, indeed, pretended, that fishes do not use their smell in the selection of food, some, it is reasonable to conclude, more so than others; but it is certainly not their principal means of discovering it. We have frequently watched trouts in a river, motionless, with their head directed against the stream, and evidently watching, not smelling, for their food; and we have seen them not only dart at flies settling on the surface, or at small fishes wandering near, but at an artificial fly which the angler has thrown within their reach; we know, too, how passing clouds, throwing a fleeting shadow on

fibres, and the appearance of their denticulations. As among other ani

mals, there is a very great difference in the magnitude of the pupil of the eye, and also of the eye itself. In one instance, that, namely, of the anableps, there is a double pupil in each eye. Fishes which reside in the depths of the ocean, from one to two or three hundred fathoms below its surface, where the light of day scarcely reaches them, or which makes at most but an obscure twilight, have the eyes large, like those of nocturnal quadrupeds or birds; while on the contrary, in such as live in mud, and burrow in the oozy slime of rivers, or low shores, the eyes are small, and often rudimental; in the myxines there are no traces of eyes. The following sketch will serve to show the

in its structure, and envelopes the optic nerve; it is of a deep red colour, but its use is quite unknown: some have imagined it to be glandular, others muscular; but it seems rather to consist of a tissue of blood vessels.

We may now notice the respiratory organs of fishes, in connexion with the heart. The respiratory organs of fishes consist essentially of the gills, or branchiæ, which are defended externally, an operculum or gill lid, capable of being opened at pleasure, and when opened, exhibiting a wide semilunar fissure, laterally seated between the head and body, on each side. In this great fissure, which freely communicates with the mouth, the gills are placed; but besides brane more or less developed, termed the true operculum, there exists a memthe gill flap, or membrana brancheostega, supported by a series of slender bones, arising from the os hyoides, or bone of the tongue: this gill flap forms the lower margin of the operculum, under which it is generally folded, and which it assists in covering the gills. Being moveable, like a fin, independently of the operculum, it may assist the mouth, in throwing a current of water over the gills, or it may effect this when the mouth is occupied in seizing food:see sketch a, the operculum, or gill lid;

general structure of the eyes as they exist in the present class of animals a a, the sclerotic coat; bb, the choroid coat; cc, the retina, expanding from the optic nerve, d; e, the vitreous humour; f, the crystalline lens, with its membranous support; 9, the aqueous humour; h, the cornea; i, the continuation of the skin covering the cornea; kk, a large body, between the choroid and the sclerotic, which is very vascular

b, the membrana brancheostega, or gill flap of the common herring. On raising this apparatus, we see beneath it the gills, of a beautiful red colour, composed of cartilaginous arches, varying, in different species, in the degree of their flexure, with their convexity posterior: this convexity is fringed with a series of vascular fibrils, set like the plumelets composing the vane of a feather; and when minutely examined, they are found to be covered with a velvet-like membrane, over which myriads of wonderfully minute blood vessels are spread, like