the science of fishes, or that part of zoology which treats of fishes. See Fish.

Fishes form the fourth class of animals in the Linnaean system. This class is there arranged into six orders, under three great divisions; none of which, however, include the cetaceous tribes, or the whale, dolphin, &c., these forming an order of the class Mammalia in the same system. See Zoology.

Mr Pennant, in his British Zoology, makes a different and very judicious arrangement, by which the cetaceae are restored to their proper rank. He distributes fish into three divisions, comprehending six orders. His divisions are, into Cetaceous, Cartilaginous, and Bony.

Div. I. CETACEOUS FISH; the characters of which are the following: No gills; an orifice on the top of the head, through which they breathe and eject water; a flat or horizontal tail; exemplified in Plate CCLI. (lower compartment), fig. 1. by the Beaked Whale, borrowed from Dale's Hist. Harw. 411. Tab. xiv.—This division comprehends three genera; the Whale, Cachalot, and Dolphin.

Div. II. CARTILAGINOUS FISH; the characters of which are: Breathing through certain apertures, generally placed on each side the neck; but in some instances beneath, in some above, and from one to seven in number on each part, except in the pipe-fish, which has only one; the muscles supported by cartilages instead of bones. Example, the Picked Dog-fish, fig. 2. a, The lateral apertures.—The genera are, the Lamprey, Skate, Shark, Fithing-frog, Sturgeon, Sun-fish, Lump-fish, Pipe-fish.

Div. III. BONY FISH; includes those whose muscles are supported by bones or spines, which breathe through gills covered or guarded by thin bony plates, open on the side, and dilatable by means of a certain row of bones on their lower part, each separated by a thin web; which bones are called the radii branchiolegi, or the gill covering rays. The tails of all the fish that form this division are placed in a situation perpendicular to the body; and this is an invariable character.

The great sections of the Bony Fish into Apodals, Thoracic, Jugular, and Abdominal, he copies from Linnaeus: who founds this system on a comparison of the ventral fins to the feet of land-animals or reptiles; and either from the want of them, or their particular situation in respect to the other fins, establishes his sections.—In order to render them perfectly intelligible, it is necessary to refer to those several organs of movement, and some other parts, in a perfect fish, or one taken out of the three last sections. In fig. 4. (the Haddock), a, is the pectoral fins; b, ventral fins; c, anal fins; d, caudal fin, or the tail; e, e, e, dorsal fins; f, bony plates that cover the gills; g, branchiostegous rays and their membranes; h, lateral or side line.

Sect. 1. APODAL: The most imperfect, wanting the ventral fins; illustrated by the Conger, fig. 3.—This also expresses the union of the dorsal and anal fins with the tail, as is found in some few fish.—Genera: The Eel, Wolf-fish, Launce, Morris, Sword-fish.

Sect. 2. JUGULAR: The ventral fins b, placed before the pectoral fins a, as in the Haddock, fig. 4.—Genera: The Dragonet, Weaver, Codfish, Blenny.

Sect. 3. THORACIC: The ventral fins a, placed beneath the pectoral fins b, as in the Father Lasher, fig. 5.—Genera: The Goby, Bull-head, Doree, Flounder, Gilt-head, Wrasse, Perch, Stickleback, Mackarel, Surmullet, Gurnard.

Sect. 4. ABDOMINAL: The ventral fins placed behind the pectoral fins, as in the Minnow, fig. 6.—Genera: The Loche, Salmon, Pike, Argentine, Athene, Mullet, Flying fish, Herring, Carp.

Naturalists observe an exceeding great degree of will-shape of dom in the structure of fishes, and in their conformity to the element in which they are to live, Most miraculously fitted for swift motion; of them have the same external form, sharp at either end, and swelling in the middle, by which they are enabled to traverse the fluid in which they reside with greater velocity and ease. This shape is in some measure imitated by men in those vessels which they design to sail with the greatest swiftness; but the progress of the swiftest sailing ship is far inferior to that of fishes. Any of the large fishes overtake a ship in full sail with the greatest ease, play round it as though it did not move at all, and can get before it at pleasure.

The chief instruments of a fish's motion have been supposed to be the fins; which in some are much more numerous than in others. A fish completely fitted for swimming with rapidity, is generally furnished with two pair of fins on the sides, and three single ones, two above, and one below. But it does not always happen that the fish which has the greatest number of fins is the swiftest swimmer. The shark is thought to be one of the swiftest fishes, and yet it has no fins on its belly; the haddock seems to be more completely fitted for motion, and yet it does not move so swiftly. It is even observable, that some fishes which have no fins at all, such as lobsters, dart forward with prodigious rapidity, by means of their tail; and the instrument of progressive motion, in all fishes, is now found to be the tail. The great use of the fins is to keep the body in equilibrium; and if the fins are cut off, the fish can still swim; but will turn upon its sides or its back, without being able to keep itself in an erect posture as before. If the fish desires to turn, a blow from from the tail sends it about in an instant; but if the tail strikes both ways, then the motion is progressive.

All fishes are furnished with a slimy glutinous matter, which defends their bodies from the immediate contact of the surrounding fluid, and which likewise, in all probability, assists their motion through the water. Beneath this, in many kinds, is found a strong covering of scales, which, like a coat of mail, defends it still more powerfully; and under that, before we come to the muscular parts of the body, lies an oily substance, which also tends to preserve the requisite warmth and vigour.

By many naturalists fishes are considered as of a nature very much inferior to land animals, whether beasts or birds. Their sense of feeling, it is thought, must be very obscure on account of the scaly coat of mail in which they are wrapped up. The sense of smelling also, it is said, they can have only in a very small degree. All fishes, indeed, have one or more nostrils; and even those that have not the holes perceptible without, yet have the bones within, properly formed for smelling. But as the air is the only medium we know proper for the distribution of odours, it cannot be supposed that these animals which reside constantly in the water can be affected by them. As to tasting, they seem to make very little distinction. The palate of most fishes is hard and bony, and consequently incapable of the powers of relishing different substances; and accordingly these voracious animals have often been observed to swallow the fisherman's plummet instead of the bait. Hearing is generally thought to be totally deficient in fishes, notwithstanding the discoveries of some anatomists who pretend to have found out the bones designed for the organ of hearing in their heads. They have no voice, it is said, to communicate with each other, and consequently have no need of an organ for hearing. Sight seems to be that sense of which they are possessed in the greatest degree; and yet even this seems obscure, if we compare it with that of other animals. The eye, in almost all fishes, is covered with the same transparent skin which covers the rest of the head, and which probably serves to defend it in the water, as they are without eyelids. The globe is more depressed anteriorly, and is furnished behind with a muscle which serves to lengthen or flatten it as there is occasion. The crystalline humour, which in quadrupeds is flat, and of the shape of a button-mould, or like a very convex lens, in fishes is quite round, or sometimes oblong like an egg. Hence it is thought that fishes are extremely near sighted; and that, even in the water, they can perceive objects only at a very small distance. Hence, say they, it is evident how far fishes are below terrestrial animals in their sensations, and consequently in their enjoyments. Even their brain, which is by some supposed to be of a size with every creature's understanding, shows that fishes are very much inferior to birds in this respect.

Others argue differently with regard to the nature of fishes.—With respect to the sense of feeling, say they, it cannot be justly argued that fishes are deficient, merely because they are covered with scales, as it is possible these scales may be ended with as great a power of sensation as we can imagine. The sense of feeling is not properly connected with softness in any organ, more than with hardness in it. A similar argument may be used with regard to smelling; for though we do not know how smells can be propagated in water, that is by no means a proof that they are not so. On the contrary, as water is found to be capable of absorbing putrid effluvia from the air, nothing is more probable than that these putrid effluvia, when mixed with the water, would affect the olfactory organs of fishes, as well as they affect ours when mixed with the air.—With regard to taste, it certainly appears, that fishes are able to distinguish their proper food from what is improper, as well as other animals. Indeed, no voracious animal seems to be endowed with much sensibility in this respect; nor would it probably be consistent with that way of promiscuously devouring every creature that comes within its reach, without which these kinds of animals could not subsist.

With respect to the hearing of fishes, it is urged, Sense of that, when kept in a pond, they may be made to hear, answer at the call of a whistle or the ringing of a bell; and they will even be terrified at any sudden and violent noise, such as thunder, the firing of guns, &c. and shrink to the bottom of the water. Among the ancients, many were of opinion that fishes had the sense of hearing, though they were by no means satisfied about the ways or passages by which they heard. Placentini afterwards discovered some bones in the head of the pike, which had very much the appearance of being organs of hearing, though he could never discover any external passages to them. Klein affirmed, from his own experiments and observations, that all fishes have the organs of hearing; and have also passages from without to these organs, though in many species they are difficult to be seen; and that even the most minute and obscure of these are capable of communicating a tremulous motion to those organs, from sounds issuing from without. This is likewise asserted by M. Geoffroy, who gives a particular description of the organs of hearing belonging to several species. These organs are a set of little bones extremely hard, and pale white, like fine porcelain, which are to be found in the heads of all fishes: The external auditory passages are very small; being scarce sufficient to admit a hog's bristle; though with care they may be distinguished in almost all fishes. It can by no means be thought that the water is an improper medium of sound, seeing daily experience shows us that sounds may be conveyed not only through water, but through the most solid bodies. It seems indeed very difficult to determine the matter by experiment. Mr Gouan, who kept some gold fishes in a vase, informs us, that whatever noise he made, he could neither terrify nor disturb them; he halloo'd as loud as he could, putting a piece of paper between his mouth and the water, to prevent the vibrations from affecting the surface, and the fishes still seemed insensible: but when the paper was removed, and the sound had its full effect on the water, the cage was then altered, and the fishes instantly sunk to the bottom. This experiment, however, or others similar to it, cannot prove that the fishes did not hear the sounds before the paper was removed; it only shows that they were not alarmed till a sensible vibration was introduced into the water. The call of a whistle may also be supposed to affect the water in a fish pond with a vibratory motion: but this certainly must be very obscure; and if fishes can be assembled in this manner, Ichthyology.

When no person is in sight, it amounts to a demonstration that they actually do hear. See Comparative Anatomy, p. 167.

The arguments used against the sight of fishes are the weakest of all. Many instances which daily occur show that fishes have a very acute sight, not only of objects in the water, but of those in the air. Their jumping out of the water in order to catch flies is an abundant proof of this; and this they will continue to do in a fine summer evening, even after it is so dark that we cannot distinguish the insects they attempt to catch.

Though fishes are formed for living entirely in the water, yet they cannot subsist without air. On this subject Mr Hawksbee made several experiments, which are recorded in the Philosophical Transactions. The fishes he employed were gudgeons; a species that are very lively in the water, and can live a considerable time out of it. Three of them were put into a glass vessel with about three pints of fresh water, which was designed as a standard to compare the others by. Into another glass, to a like quantity of water, were put three more gudgeons, and thus the water filled the glass to the very brim. Upon this he screwed down a brass plate with a leather below, to prevent any communication between the water and the external air; and, that it might the better resemble a pond frozen over, he suffered as little air as possible to remain on the surface of the water. A third glass had the same quantity of water put into it; which, first by boiling, and then by continuing it a whole night in vacuo, was purged of its air as well as possible; and into this also were put three gudgeons. In about half an hour, the fishes in the water from whence the air had been exhausted, began to discover some signs of uneasiness by a more than ordinary motion in their mouths and gills. Those who had no communication with the external air, would at this time also frequently ascend to the top, and suddenly swim down again; and in this state they continued for a considerable time, without any sensible alteration. About five hours after this observation, the fishes in the exhausted water were not so active as before, upon shaking the glass which contained them. In three hours more, the included fishes lay all at the bottom of the glass with their bellies upwards; nor could they be made to shake their fins or tail by any motion given to the glass. They had a motion with their mouths, however, which showed that they were not perfectly dead. On uncovering the vessel which contained them, they revived in two or three hours, and were perfectly well next morning; at which time those in the exhausted water were also recovered. The vessel containing these last being put under the receiver of an air-pump, and the air exhausted, they all instantly died. They continued at top while the air remained exhausted, but sunk to the bottom on the admission of the atmosphere.

The use of air to fishes is very difficult to be explained; and indeed their method of obtaining the supply of which they stand constantly in need, is not easily accounted for. The motion of the gills in fishes is certainly analogous to our breathing, and seems to be the operation by which they separate the air from the water. Their manner of breathing is as follows. The fish first takes a quantity of water by the mouth, which is driven to the gills; these close, and keep the water which is swallowed from returning by the mouth, while the bony covering of the gills prevents it from going through them till the animal has drawn the proper quantity of air from it; then the bony covers open, and give it a free passage; by which means also the gills are again opened, and admit a fresh quantity of water. If the fish is prevented from the free play of its gills, it soon falls into convulsions, and dies. But though this is a pretty plausible explanation of the respiration of fishes, it remains a difficulty not easily solved what is done with this air. There seems to be no receptacle for containing it, except the air bladder or swim; which, by the generality of modern philosophers, is defined not to answer any vital purpose, but only to enable the fish to rise or sink at pleasure.

The air-bladder is a bag filled with air, composed of the use sometimes of one, sometimes of two, and sometimes of the air-three divisions, situated towards the back of the fish, bladder in and opening into the maw or the gullet. The use of this in raising or depressing the fish, is proved by the following experiment. A carp being put into the air-pump, and the air exhausted, the bladder is said to burst by the expansion of the air contained in it; after which, the fish can no more rise to the top, but ever afterwards crawls at the bottom. The same thing also happens when the air-bladder is pricked or wounded in such a manner as to let the air out; in these cases also the fish continues at the bottom, without a possibility of rising to the top. From this it is inferred, that the use of the air-bladder is, by swelling at the will of the animal, to increase the surface of the fish's body, and thence diminishing its specific gravity, to enable it to rise to the top of the water, and to keep there at pleasure. On the contrary, when the fish wants to descend, it is thought to contract the air-bladder; and being thus rendered specifically heavier, it descends to the bottom.

The ancients were of opinion, that the air-bladder in fishes served for some purposes essentially necessary to life; and Dr Priestley also conjectures, that the raising or depressing the fish is not the only use of these air-bladders, but that they also may serve some other purposes in the economy of fishes. There are many arguments indeed to be used on this side of the question; the most conclusive of which is, that all the cartilaginous kind of fishes want air-bladders, and yet they rise to the top or sink to the bottom of the water without any difficulty; and though most of the cold-kind have air-bladders, yet they cannot raise themselves in the water without great difficulty.

Fishes are remarkable for their longevity. "Most Longevity of the disorders incident to mankind (says Bacon) arise of fishes from the changes and alterations in the atmosphere; but fishes reside in an element little subject to change; theirs is an uniform existence; their movements are without effort, and their life without labour. Their bones, also, which are united by cartilages, admit of indefinite extension; and the different sizes of animals of the same kind, among fishes, is very various. They still keep growing; their bodies, instead of suffering the rigidity of age, which is the cause of the natural decay of land-animals, still continue increasing with fresh supplies; and as the body grows, the conduits of of life furnish their stores in greater abundance. How long a fish, that seems to have scarce any bounds put to its growth, continues to live, is not ascertained; perhaps the life of a man would not be sufficient to measure that of the smallest. —There have been two methods fallen upon for determining the age of fishes; the one is by the circles of the scales, the other by the transverse section of the back bone. When a fish's scale is examined by a microscope, it is found to consist of a number of circles one within another, in some measure resembling those which appear on the transverse section of a tree, and is supposed to give the same information. For, as in trees, we can tell their age by the number of their circles; so, in fishes, we can tell theirs by the number of circles in every scale, reckoning one ring for every year of the animal's existence.—The age of fishes that want scales may be known by the other method, namely, by separating the joints of the back-bone, and then minutely observing the number of rings which the surface, where it was joined, exhibits.

Fishes are, in general, the most voracious animals in nature. In most of them, the mouth is placed next the mouth; and, though possessed of no sensible heat, is endowed with a very surprising faculty of digestion. Its digestive power seems, in some measure, to increase in proportion to the quantity of food with which the fish is supplied. A single pike has been known to devour 100 roaches in three days. Whatever is possessed of life, seems to be the most desirable prey for fishes. Some that have very small mouths, feed upon worms, and the spawn of other fish; others, whose mouths are larger, seek larger prey; it matters not what kind, whether of their own species, or any other. Those with the largest mouths pursue almost everything that hath life; and often meeting each other in fierce opposition, the fish with the largest swallow comes off with the victory, and devours its antagonist.—As a counterbalance to this great voracity, however, fishes are incredibly prolific. Some bring forth their young alive, others produce only eggs; the former are rather the least fruitful; yet even these produce in great abundance. The viviparous blenny, for instance, brings forth 200 or 300 at a time. Those which produce eggs, which they are obliged to leave to chance, either on the bottom where the water is shallow, or floating on the surface where it is deeper, are all much more prolific, and seem to proportion their flock to the danger there is of consumption.—Lewenhoeck affirms us, that the cod spawns above nine millions in a season. The flounder commonly produces above one million, and the mackerel above 500,000. Scarce one in 100 of these eggs, however, brings forth an animal; they are devoured by all the lesser fish that frequent the shores, by water-fowl in shallow waters, and by the larger fishes in deep waters. Such a prodigious increase, if permitted to come to maturity, would overstock nature; even the ocean itself would not be able to contain, much less provide for, one half of its inhabitants. But two wise purposes are answered by this amazing increase; it preserves the species in the midst of numberless enemies, and serves to furnish the rest with a subsistence adapted to their nature.

With respect to the generation of many kinds of fishes, the common opinion is, that the female deposits her spawn or eggs, and that the male afterwards ejects his sperm or male semen upon it in the water. The want of the organs of generation in fishes gives an apparent probability to this; but it is strenuously opposed by Linnæus. He affirms, that there can be no possibility of impregnating the eggs of any animal out of its body. To confirm this, the general course of nature, not only in birds, quadrupeds, and insects, but even in the vegetable world, has been called into his assistance, as proving that all impregnation is performed while the egg is in the body of its parent: and he supplies the want of the organs of generation by a very strange process, affirming, that the males eject their semen always some days before the females deposit their ova or spawn; and that the females swallow this, and thus have their eggs impregnated with it. He says, that he has frequently seen, at this time, three or four females gathered about a male, and greedily snatching up into their mouths the semen he ejects. He mentions some of the eels, some perch, and some of the cyprini, in which he had seen this process. But see Comparative Anatomy, no 154.

Many opinions have been started in order to account how it happens that fishes are found in pools, and ditches, on high mountains, and elsewhere. But Gmelin observes, that the duck-kind swallow the eggs of fishes; and that some of these eggs go down, and come out of their bodies unhurt, and so are propagated just in the same manner as has been observed of plants.

For a more particular view of the structure of fishes, see Comparative Anatomy, no 146—167.