FISHES, Pisces, the fourth—or, according to Milne-Edwards and some other natural ists (see AMPRIBIA), the fifth—class of vertebrated animals; consisting of creatures which live in water, and accordingly breathe by gills (bronchia), and not, at any stage of their existence, by lungs. In number—both of individuals and of different kinds—they are supposed to exceed all the other classes of vertebrate animals put together. Even the water of hot springs and the pools of caverns have their peculiar F., and some of these are only known as thrown out with torrents of muddy water by volcanoes.
The form of F. is generally adapted to easy and rapid progression through water, being more or less nearly that of a spindle, swelling, in the middle,.andltapering towards the extremities; the outline unangular, and the surface smooth. But exceptions to this rule are numerous; and some provided with other means of seeking their food, or of preservation from their enemies, exhibit the greatest possible departures from the ordin ary shape: some are globe-shaped, some have a most irregular and angular outline, many are much elongated, as eels; and others are compressed and flattened, as flounders.
The bones of F. differ much in their structure from those of other vertebrate animals: they are less dense and compact, and when their ossification is perfect, remain separte, as in the early embryotic state of the mammalia. The bones of the sub-class of cartilaginous F. (q.v.), however, never become properly ossified. The bones of F. generally contain a smaller proportion of earthy matter than those of other verte brate animals, and their cartilaginous basis contains no gelatine strictly so called. The typical character of the vertebrate skeleton is, however, maintained, although modified; and many of the hones—a great majority, for example, of those of the head—are evi dently homologous with those of quadrupeds and of man. There is no neck, and the vertebrm are distinguishable only into abdominal and caudal. The vertebra: are concave at each end, and pierced in the middle, the hollow space being occupied with a gelatin ous substance. Spinous processes, sometimes short, sometimes long, extend upwards and downwards from the vertebrm to support the muscles. F. also generally have ribs, connected with the abdominal vertebrae; and in many, an additional set of small bones (epipleural spines) connected with the ribs, and arising from near the base of the ribs, extends outwards and backwards through the lateral muscles. The four limbs which belong to the typical structure of vertebrate animals, assume in F. the form of fins (q.v.), and are generally, although not always, all present, the first pair being the pectoral, the second pair the ventral fins. In some F., the ventral fins, answering to the bind-feet of quadrupeds, are actually further forward than the pectoral fins, and are then called jugular fins. In some, as the common eel, the ventral fins are wanting; in some, as the murcena, there are neither pectoral nor ventral fins. Connected with these fins are bones, which show that they represent the limbs of other vertebrate animals. F. have, however, also other fins not so closely connected as these with the internal (endo-) skeleton, and not placed like them in pairs towards the sides, but vertically on the middle (mesh') line; one or more (dorsal) on the back; one or more (anal) on the opposite or ventral aspect, behind the anus; and one (caudal) at the extremity of the tail. The caudal fin is in general the principal organ of locomotion, and most of the muscles of the body combine to give great energy to its strokes, great part of the body moving with it, and the vertebrm with their processes being so framed as to admit great freedom of lateral, and scarcely any vertical motion. The pectoral and ventral fins seem to serve chiefly for balancing the body, and guiding and staying its motion; the dorsal and anal fins, like the keel of a ship, for keeping it in its proper position. All the vertical fins are supported by bones which do not join those of the internal skeleton, but are thickest at the skin, from which they penetrate towards the vertebrm, being interposed between the spinous processes of the vertebral column: Several of the last caudal vertebrm are generally very short and combined, and the interposed spines which support the caudal fin converge towards them. The rays of tins are either pointed
bones (spines)—sometimes prolonged beyond the membrane, and forming defensive weapons—or they are cartilaginous and jointed, in which case they often also branch near their summit. The caudal fin never has any other than these soft rays, and many F. have no other in any of their fins. A few F., belonging to different families, have the pectoral fins developed to an unusual degree, so as to make them capable of sup porting short flights in the air (see FLYING Fists and FLYING GurnsAnD); and a few are capable of employing their fins as organs of locomotion in a very different way, creeping along the ground, or hopping among the weeds and stones of the shore. • ' The heart of F. consists only of one auricle and one ventricle, receiving venous blood only, and sending it to the gills, where, being oxygenated, it passes into the greater or systemic circulation by the dorsal vessel. See CIRCULATION. In most F., there is, close to the heart, a thick bulb or muscular swelling of the great artery which conveys the blood from the heart to the gills, and which assists in the blood, being furnished with valves to prevent its regurgitation into the heart; and this bulb and its valves exhibit varieties admirably characteristic of different natural groups, much founded upon in the system of Muller and Owen. The blood of F. is red; its corpuscles are oval and of considerable size, but in general not very numerous. F. con sume little oxygen in respiration, and are cold-blooded animals, having in general a tem perature little elevated above that of the water in which they live; although there are some singular exceptions to this rule, as the tumiy, sword-fish, etc., which, having a comparatively high temperature, have also redder blood with more numerous corpuscles. The oxygen appropriated by means of the gills in respiration is not obtained by decom position of water, but from the air which is mixed in it, and hence the necessity of an aquarium; hence also we perceive one of the benefits resulting from the agitation of the ocean and of lakes by winds. Some F. require a greater supply of air than they can easily obtain from the water, and frequently come to the surface to breathe. F. taken out of the water die from want of breath, in consequence of the drying up of the fine fringes of the gills; and those which are capable of subsisting longer out of water than others, have generally small gill openings, not so freely admit tang the air to dry the gills, whilst a few are provided with receptacles for water to keep them moist. See ANABASID/E. • The gills of F. are situated at the back part of the sides of the head, and consist of a multitude of very vascular membranous plants, which are generally in double fringe-like rows fixed by the base only, and simple, although in a few F. they are feathery, and in the greater number of cartilaginous fishes (q.v.), they are fixed both by their external and their internal edges, or consist of mere folds of membrane attached to the surface of the gill-cavities. In general, there are four gills on each side; the number is greater in some of the cartilaginous fishes. In osseous F., the gill-plates are attached to the external edge of the branchial arches, bony arches connected with the hyoid bone or bone of the tongue—which is unusually developed in F.—and with the base of the skull, time connection at both ends being effected by intervening small bones, and the whole forming a complicated system; whilst the cavity containing the gills, on each .side of the head, is covered by a bogy plate, the gill-lid, gill-cover, or operculum, with two subordinate pieces, called the sub-operculum and inter-opercuium, articulated on the temporal bone, and playing on the'pre-operculum, a bony plate placed before them in the head. .It is by the motion of these bony plates that the water is expelled which is taken in by the mouth, and which, after passing amongst the gills, and supplying them with air, passes out by the gill-orifices at the back of the head. Besides these opercular plates or bones, a series of flattened rays, connecting them with the bone of the tongue, and called the bra nchiostegal rays, aid in forming the gill cavi ties. In the branchiostegal rays, distinctive characters of F. are often found.