PISCES. (Eng. Fishes ; Ft. Poissons; Germ. Fische. )—The lowest class of the ver tebrate division of the animal kingdom, em bracing numerous oviparous races of beings fitted by their organization to live only in water, and consequently they are the appro priate inhabitants of the ocean and of inland streams and lakes. Being strictly aquatic in their habits, Fishes respire through the medium of the element in which they live by means of gills or branclii, that are connected with a framework of bony or cartilaginous arches situa ted on the sides of the neck, to which the water obtains free access, generally passing in at the mouth and escaping through lateral openings situated behind the head. Their heart is hilocu lar, and consists of an auricle and ventricle, which, receiving the venous blood from the sys tern, propel it over the respiratory surface, whence it is collected into an arterial trunk, the aorta, by which it is distributed over the body without the intervention of a systemic heart. Their blood is of very low temperature, and their bodies are generally covered with scales of va rious kinds, whereby they are preserved from maceration in the surrounding water, and fitted to glide smoothly through the fluid medium wherein they live. Their principal instrument of progression is their tail, which is generally expanded into a broad fin, that strikes the water by alternate lateral movements. Besides this caudal fin others are frequently met with situated along the median line of the body, to which the names of dorsal and anal fins have been appropriated accordingly as they are situa ted upon the back or behind the anal outlet of the body. The position of these azygos fins is vertical, and their use to a fish is similar to that of the keel or of the helm to a ship. The repre sentatives of the anterior and posterior extremi ties of other Vertebrata likewise take the form of fins, and are only fitted for progression in the water : these are generally four in number, namely, the two pectoral fits, which represent the anterior extremities; and the two ventral fins, corresponding, with the posterior limbs of Quadrupeds. Great variety is met with both in the number and position of these locomotive members ; generally all four are present; fre quently one pair is deficient, and sometimes they are altogether wanting. In situation they likewise vary, more especially the ventral pair, which in some races, instead of being behind, are situated in front of the abdomen, in con nection with the scapular apparatus, and even anterior to the pectoral fins.
In the construction of their cerebral system Fishes evidently stand lowest in the vertebrate scale, and every part of their economy indicates their inferiority to Reptiles, Birds, and Mam mals.
The general attributes of Fishes and their relative position in the animal scale are so well laid down by their great modern historian, Cuvier, that it would be presumptuous not to give his own words.
" Breathing by the medium of water, that is to say, only profiting by the small quan tity of oxygen contained in the air mixed with the water, their blood remains cold; their vita lity, the energy of their senses and movements are less than iu Mammalia and Birds. Thus their brain, although similar in composition, is proportionally much smaller, and their external organs of sense not calculated to impress upon it powerful sensations." " Fishes are in fact, of all th e Vertebrata, those which give the least apparent evidence of sensi bility. Having no elastic air at their disposal, they are dumb, or nearly so, and all the senti ments which voice awakens or entertains they are strangers to. Their eyes are as it were mo tionless, their face bony and fixed, their limbs incapable of flexion and moving as one piece, leaving no play to their physiognomy, no ex pression to their feelings. Their ear, enclosed entirely in the cranium, without external concha, or internal cochlea, composed only of some sacs and membranous canals, can hardly suffice to distinguish the most striking sounds, and, moreover, they have little use for the sense of hearing, condemned to live in the empire of silence, where every thing around is mute." " Even their sight in the depths which they frequent could have little exercise, if most of them had not, in the size of their eyes, a means of compensation for the feebleness of the light; but even in these the eye hardly changes its direction, still less by altering its dimensions can it accommodate itself to the distances of objects. The iris never dilates or contracts, and the pupil remains the same in all intensities of illumination. No tear ever vvaters the eye—no eyelid vvipes or protects it—it is in the Fish but a feeble representative of this organ, so beau tiful, so lively, and so animated in the higher classes of animals."
" Being only able to support itself by pursuing a prey which itself swims more or less rapidly, having no means of seizing it but by swallow ing, a delicate perception of savours would have been useless, if nature bad bestowed it; but their tongue almost motionless, often entirely bony or coated with dental plates, and only furnished with slender nerves, and these few in number, shews us that this organ also is as obtuse as its little use would lead us to ima gine it." " Their smell even cannot be exercised so continually as in animals which respire air and have their nostrils constantly tmversed by odorous vapours." " Lastly, their touch, almost annihilated at the surface of their body by the scales which clothe them, and in their limbs by the want of flexibility in their rays, and the nature of the membranes investing them, is confined to the ends of their lips, and even these in some are osseous and insensible." " Thus the external senses of Fishes give them few lively and distinct impressions. Surround ing nature cannot affect them but in a confused manner; their pleasures are little varied, and they have no painful impressions from without but such as are produced by wounds.'' " Their continual need, which, except in the breeding season, alone occupies and guides them, is to assuage the internal feeling of hunger, to devour almost all that they can. To pursue a prey or to escape from a pursuer niakes the occupation of their life ; it is this which determines their choice of the different situations which they inhabit; it is the prin cipal cause of the varieiy of their forms and of the special instincts or artifices which nature has granted to some of the species." " Vicissitudes of temperature affect them little, not only because these are less in the ele ment which they inhabit than in our atmosphere, but because their bodies taking the surrounding temperature the contrast of external cold and internal heat scarcely exists in their case. Thus the seasons are not so exclusively the regulators of their migration and propagation as amongst Quadrupeds or more especially Birds. Many Fishes spawn in winter; it is towards autumn that hemng,s come out of the north to shed upon our coast their spawn and milt. It is in the north that the most astonishing fecundity is witnessed, if not in variety of species, at least in individuals ; and in no other seas do we find anything approaching to the countless myriads of herrings and cod which attract whole fleets to the northem fisheries." " The loves of Fishes are cold as themselves; they only indicate individual need. Scarcely is it permitted to a few species that the two sexes should pair and enjoy pleasure together; in the rest the males pursue the eggs rather than seek the females ; they are reduced to impregnate eg,gs the mother of which is un known, and whose produce they will never see. The pleasures of maternity are equally un known to most species; a small number only carry their eggs with them for a short time ; with few exceptions Fishes have no nest to build and no young to nourish : in a word, even to the last details, their economy contrasts diame trically with that of Birds." In no class of the animal kingdom do we find such diversity of form as in that of Fishes. Some amongst them are perfectly spherical, as the Diodons. Others are discoidal, or flat and circular, and this shape may be produced by two very different conditions, resulting either from an excessive narrowing or inordinate ex pansion of the two sides of the body. In the first case it is compressed and much elevated, as in Vomer and Orthagoriscus, while in the second case it is much depressed, flattened, and very broad, as in the Skates. Other species are oval, more or less elongated and slightly compressed laterally, such as Carp, Trout, &c., which is the most ordinary shape. Neverthe less when these become extended longitudi nally (as in the Pikes for example), we are insensibly conducted by all intermediate grada tions of form to the cylindrical Eels, or to com pressed and riband-shaped Fishes, such as Cepola. Perhaps the most remarkably shaped Fishes are those whose bodies are bounded by nearly flat surfaces, and which circumscribe angular figures, such as triangles, squares, pen tagons, hexagons, &c.,( Ostracion,Syrignathia.) There are even certain genera in which the two sides are not symmetrical, one being flattened and the other vaulted, and in these races even the bones of the cranium are so disproportioned that both eyes are turned to the same side of the animal ( Pleuronectida).
The following arrangement, being a modifi cation of the classification proposed by Cuvier, will facilitate our investigations relative to the anatomy of the numerous members of this extensive class.