The eyes have neither true lids nor lachrymal apparatus; the pupil is large and permanently open, the lens is spherical, and the flat cornea is covered by the skin. Fishes are very voracious, most of them living on animal food, and swallowing indiscriminately anything of this kind which comes in their way; some genera, like the lamprey eels, live upon the juices of other fish, and the mouth is provided with circular cartilages, fleshy disks, teeth, and a piston-like tongue, which enable them to adhere to any surface. The intestinal canal is short and simple, and digestion is rapidly performed, and their increase in size is remarkably affected by the nature and abundance of their food; their limits as to size and the natural duration of life are very little known in the great majority of species.-The blood of fishes is red, and the globules are elliptical and of considerable size. The heart is placed under the throat in a cavity separated from the abdomen by a kind of diaphragm, protected by the pharyngeal bones above, the branchial arches on the sides, and generally by the scapular arch behind; it consists of a venous sinus, auricle, ventricle, and bulb; all these cavities circulate venous blood, and therefore physiologically correspond to the right side of the mammalian heart, though Owen says that the heart of fishes with the muscular branchial artery is the true homologue of the left auricle, ventricle, and aorta of higher vertebrates, tracing the complication of the organ synthetically; the auricle and ventricle, however, are alone proper to the heart itself, the sinus being the termination of the venous system, and the bulb an addition to the pulmonary artery; these four compartments, therefore, are not like the four divisions of the human heart, but succeed each other in a linear series.

The circulation is double, that of the system at large and that of the branchiae being complete and distinct, and there is also an abdoininal circulation terminating at the liver; the peculiar character is that the branchial circulation alone is provided with a propelling cavity or heart, the branchial veins changing into arteries without any intermediate left auricle and ventricle. The venous sinus receives the blood from the general system, after the manner of vena) cava); it is not usually situated within the pericardium. The auricle, when distended, is larger in proportion to the ventricle than in the higher vertebrates; its walls are membranous, with thin muscular fasciculi, and its simple cavity communicates with the ventricle by a single opening guarded by free semilunar valves, two to four in number. The ventricle, usually a four-sided pyramid, is very muscular, and its fibres are redder than those of any other part of the system; its cavity is simple, the auricular valve generally free and without chorda) tendineae, and its opening into the bulb provided with two or four semilunar valves. The contractile bulbus arteriosus is provided in the ganoids and plagiostomes with several rows of valves, and its muscular walls are distinct from those of the ventricle.

The immediate force of the heart's action is applied through the continuation of the bulb into the branchial artery, which is generally short, and is divided into lateral branches going to the gills; the blood, which has become ar-terialized by its subjection to the air contained in the respired water, is carried along the returning vessels into the branchial veins, the analogues of the pulmonary veins of man; the four on each side form the aortic circle from which the pure blood is sent over the system through the carotids and the aorta and its branches; the blood of the chylopoietic viscera passes through the liver before entering the great sinus. Though all the blood passes through the branchial apparatus, it traverses the heart but once.-Respiration is effected by means of the innumerable vascular lamellae and tufts attached to the external edge of the branchial arches; these are generally four on each side, each composed of two rows of fringes; in most cartilaginous fishes there are five, and in the lamprey seven; in the last fish there is a canal from the mouth to the respiratory cavity, resembling a trachea.

Fishes consume but a small amount of oxygen, but some, not content with that contained in the water, come to the surface occasionally to swallow air; they perish soon out of water in proportion to the quickness with which the gills become dry, asphyxia being produced not by the want of oxygen directly, but because the blood cannot circulate in them properly unless sustained and kept soft by water. Though fishes produce little heat, some possess the singular faculty of generating and discharging electricity. (See Electric Fishes.)-Fishes reproduce by means of eggs, the number of which in some species amounts to hundreds of thousands; these have generally only a mucilaginous envelope, and are fecundated after being laid; a few enjoy sexual congress, and are ovovivipa-rous and viviparous, but the young are almost always left to themselves as soon as born. It is owing to the simultaneous development of great numbers of eggs deposited in the same locality, and to the instinct possessed by some species to keep in company, that fish occur in what are called banks and schools; these schools, composed of individuals kept together only by similarity of food and habits, and in which each one looks out for himself without regard to the wants of the rest, make long migrations from the sea to the rivers and back again, and from one favorite locality to another.

At the time of laying the eggs, the migrating species generally approach the shores, and ascend rivers, often coming thousands of miles; year after year, at the same season, the fish appear in immense numbers. The migrations of the herrings, salmon, shad, smelt, mackerel, etc, afford well known instances of these phenomena. All fishes are of distinct sex. The testes vary much in form in the osseous fishes, and are remarkable for their enormous development in the breeding season, when they are called milt or soft roe. The ovaries in most osseous fishes are two elongated sacs, closed anteriorly, and produced posteriorly into short, straight, and wide oviducts, which coalesce before reaching the cloaca; the greatly developed ova are called the roe. There are several interesting points in connection with the development of fishes which will be better introduced here than in special articles. In most fishes it has been already stated that the exclusion of the ova or roe precedes fecundation, and that in a few (the sharks and rays especially) the ova are fecundated before exclusion; when the embryonic membranes contract no adhesion to the uterine walls, the fish is called ovo-viviparous, and in such the embryo escapes from the egg before it quits the parent, while in the ovipara the ovum is expelled while the embryo is contained in it; when adhesion takes place by vascular interlacements, the species is said to be viviparous; the great difference between viviparous fishes and mammals is, that in the former the rupture of the membranes takes place long before birth, while in the latter this occurs at the moment of exclusion.