PISCES, in natural history, is the fourth class in the Linmean system, consisting of five orders, viz.
Abdominales Jugulares Apodes Thoracici Carulaginii The class is described as having incum bent jaws ; eggs without white ; organs of sense ; for covering, imbricate scales ; fins for they swim in water, and smack. The several orders and other matters relative to fishes, have been treat ed of in the article ICHTHYOLOGY, and in the several parts of the Dictionary, in the alphabetical order of the genera, Sac. To this article we have referred, intending to give tinder it a brief account of the functions of the several fishes. Of these the most important is respiration, which is performed by means of gills, which supply the place of lungs. Air is equally necessary to the existence of fish, as it is to other animals. In general, a fish first re ceives a quantity of water by the mouth, from which it is driven to the gills; these close, and prevent the water from return ing by the mouth, at the same time that their bony covering prevents it from pass ing through them, until the proper quan tity of air has been extracted from it.— The covers then open, and give it a free passage ; by which means the gills are again opened, and admit a fresh body of water. This process, in fishes, as breath ing in the human subject, is carried on during sleep, and is repeated about twenty five times in a minute ; and the necessity of it is evinced from the circumstance of fish being certainly killed in water, from which air is taken away by means of the air-pump, or excluded by very severe frost. Should the free play of the gills be even suspended, or their covers kept from moving, by a string tied round them, the fish would fall into convulsions, and die in a few minutes. It is said, likewise, that though the branchial apparatus be comprised in a small compass, its sur face, when fully extended, would occupy many square feet ; a fact, that may con vince the most sceptical, of the number less convolutions and ramifications in which the included water is elaborated and attenuated, in the course of giving out its air in the respiratory process.
Fishes have the organs of sense, some of them probably in a very high degree, and others imperfectly ; of the latter kind are the senses of touch and of taste : but the sense of hearing has now been corn. pletely ascertained, which was long doubted, and by some physiologists de nied : the organ is contained in the cavity of the head ; it was discovered by Profes sor Camper, who remarks, that " fish per ceive sound, but sound peculiar to the watery element." This organ has been
observed and described by Mr. Hunter, in the Philosophical Transactions, who has likewise ascertained that its structure varies in different species. And Dr. Shaw, in his "Introduction to the Natu ral History of Fishes," Vol. IV. Part I. observes, that " Fishes, particularly of the skate kind, have a bag at some dis tance behind the eyes, which contains a fluid, and a soft cretaceous substance, and supplies the place of the vestibule and cochlea: there is a nerve distributed up on it similar to the portio mollis in man : they have semicircular canals, which are filled with a fluid, and communicate with the bag ; they have likewise a meatus ex. ternus, which leads to the internal ear. The cod-fish, and others of the same shape, have an organ of hearing some what similar to the former, but instead of a soft substance contained in the bag, there is a hard cretaceous stone." From the same work we shall transcribe the observations on the sense of smelling and that of sight.
"The organ of smelling is large, and the animals have a power of contracting and dilating the entry to it as they have occasion. It seems to be mostly by their acute smell that they discover their food, for their tongue seems not to have been designed for a very nice sensation, being of a pretty firm cartilaginous substance ; and common experience evinces that their sight is not of so much use to them as their smell in searching for their nour ishment. 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 plaln from ob serving, 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 efflu via, it shall have the same effect as for merly. Now it is certain, that • had the animals discovered this bait with their eyes, they would have come equally 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 them selves to be carried down with the stream, that they may ascend again 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 strong er sensation.