These classes have been variously subdivided, and the reader is referred to the article Ichthyology for the numerous classifications from Artedi to Agassiz. The anus may open far behind the ventrals, move forward with them, and in their absence be situated even under the throat, as in sternarchus; the jaws may be armed with different kinds of teeth, which often exist also on the tongue and various parts of the mouth and throat; the lips may be provided with sensitive barbels as in the horn pout, or with fleshy appendages as in the sea raven (hemitripterus). The skin may be nearly naked or covered with very small scales; the scales may be rough grains as in the sharks, thick plates as in the sturgeon, a smooth enamelled coat of mail as in the lepi-dosteus, smooth as in the herring, or serrated as in the perch. Along the side of the body is the lateral line, formed by a series of pores, the outlets of the muciparous glands; this line extends from the head to the caudal fin, generally at the mid height of the body, nearer the back in some fishes than in others, sometimes ceasing long before the region of the tail, and occasionally multiple; the scales along this line are arched, notched, or perforated for the protection of the ducts; they are sometimes larger or smaller than the rest, and may be the only ones present; they often have strange forms and armatures.

In various parts of the body, but especially about the head, are numerous pores, or water tubes, by which water is introduced into the system, even into the circulation; some are situated along the lateral line. The tissue of the fish skeleton is either cartilage, fibro-cartilage, or bone; the first is found in the sharks and rays, the second in the sun-fish (orthagoriscus) and angler (lophius), and the last in common fishes; the chemical composition is that of other vertebrates, principally the phosphate and carbonate of lime. The osteology of the head, branchial apparatus, trunk, and limbs has been already given as fully as the limits of this work will allow, in the article Comparative Anatomy (vol. v., p. 172); for further details see Cuvier and Valenciennes, vol. i., and Owen on "Fishes."-Most fishes are quick in their movements; the salmon, for instance, can swim at the rate of 40 ft. in a second, and can with ease pass over 20 to 25 m. in an hour; progression is effected by lateral strokes of the water by the alternate flexions of the tail and trunk; the manner in which the vertebrae are connected allows easy motion of the spine from side to side, and the muscles destined to move it are so largely developed as to form the principal bulk of the body; while the vertical tins increase the amount of oar-like surface for purposes of locomotion, the pectorals and ventrals keep the fish in an upright position, and assist in directing its course; the movements of the gill covers, by forcing backward the water which is passing between them, contribute to propel the fish forward.

In the pipe fish {syngnathus) the dorsal fin in its vibration resembles that of the screw of a steam propeller, and, with a similar action of the tail, causes a forward or backward motion without any apparent movement of the body; the nice adjustment of the movements of the tins of the pickerel, so that while every ray seems in action the fish is perfectly stationary, must have been noticed by every angler. The movements of fishes in a vertical direction are greatly assisted by the swimming or air bladder, which, though anatomically a rudimentary lung, by the air which it secretes enables those that have it to rise or fall in the water by compression or extension exercised by the ribs; it is placed in the abdomen under the spine, and communicates often with the oesophagus or stomach; the air is a product of secretion, and its containing reservoir is sometimes a shut sac; it is often wanting in some species of a genus when others possess it, and is generally very small or absent in ground fishes, such as skates and turbots; in some cases it is considerably vascular, resembling very much a pulmonary sac.

The muscles of fishes are generally pale and comparatively soft, divided into parallel layers by aponeurotic laminae; the flavor and odor are very different from those of flesh, and the gases of decomposition are much more fetid. Some fishes have a singular apparatus by which they adhere to other bodies, animate or inanimate; in the remora, of the genus echeneis, there is a flattened disk on the top of the head, composed of movable cartilaginous plates, by which it fixes itself to stones or the bodies of other fishes; in the lump fish and other discoboli, the ventrals are arranged to act as suckers for attaching them to various substances; the lamprey eel {petromyzon) also attaches itself by the mouth to stones and fishes. Referring the reader to Comparative Anatomy for details on the nervous system, the organs of sense, the scales, and the digestive apparatus, only general points of interest need be mentioned here. The cavity of the skull is very small compared to the size of the body, and the brain is far from filling it, a considerable space being occupied by a spongy fatty substance; the lobes are placed one behind the other in the following order from before backward: olfactory or lobes of smell, the cerebral hemispheres, the optic or lobes of vision, and the cerebellum.

From the scaly covering of their skin, the sense of touch must be obtuse, and the lips are their only prehensile and principal tactile organs, with the exception of the barbels and other appendages above alluded to. The corneous, slightly movable, and often tooth-armed tongue receives but few nerves, and cannot be the seat of any sense worthy of the name of taste; and moreover, the food does not remain long enough in the mouth for any exercise of this sense. The olfactory apparatus is more complicated, but it is traversed neither by air nor the water used in respiration; the nasal cavities do not communicate with the mouth. The ear, almost always entirely within the cranium, on the sides of the brain, consists essentially of a vestibule and three semicircular canals, which receive the vibrations of the integuments and cranial walls; there is rarely anything that can be called external ear, drum, or tympanic cavity; loud, sudden, and strange sounds frighten fishes, as the experience of every fisherman tells him; in ancient, and even in modern times, they have been taught to come and receive food at the tinkle of a bell, or the pronunciation of pet names.