SENSE ORGANS. Unequally scattered over the body of fishes there are the so-called 'end buds'— modifications of the epidermis. In structure these sense organs largely resemble taste buds, which in the vertebrates above fishes are re stricted to the mouth-eavity. In fishes these `end buds' are probably also taste organs, since it has been shown that a fish can taste with its skin. Besides these there are other aggregations of sense cells, probably tactile in function. Situ ated within longitudinal grooves or pits are cells, probably largely tactile in function, known as the lateral-line organs. These grooves open by definite pores to the surface. There is usually one series of such along each side, known as the 'lateral line,' but there may also be de veloped a more or less complicated system of grooves on the head. Many fishes have filamen tous appendages, more or less definitely arranged around the mouth and nose, known as barbels. Cave-dwelling fishes, which have lost their power of sight, have strongly developed tactile papillae on the head. The organs of smell are a pair of pits in the skin at the anterior do•sum of the head, lined with sense cells. There are no internal nares except in the Dipnoi, but the pits open to the surface by the external nares, each more or less completely divided into two, to permit the water to enter one, bathe the sense surface, and escape by the other. Fishes have no external and middle ear, but merely the inner, consisting of the semicircular canals, with their ampulhe, a sacculus, and utrieulus. The otoliths are large. Various experiments point to the conclusion that the ear in fishes is merely an organ of equilib rium. The eyes have the usual structure of the vertebrate eye. The accessory organs, like the lids and lachrymal glands, are poorly de veloped. Eyes may he absent in cave and deep sea forms. See NERVOUS SYSTEM. EVOLUTION OF.
The diyestior system consists, as in other vertebrates, of the alimentary canal. with its
more or less definitely marked divisions (mouth, pharynx, gullet, stomach, and intestine), and its glands, liver, gall-bladder, and the pancreas. The mouth and its teeth present the greatest variety in form and arrangement. The pharynx opens to the surface by the gill-clefts as above indi cated. The gullet and stomach vary with the food habits of the fish. Predatory fishes swal low and stow away large objects. An extreme instance is the deep-sea fish Chiasniodon niger, which has been taken with a fish in its distensible stomach larger than itself. At the junction of the stomach and intestine, in ganoids and in nearly all teleosts, are given off a number of blind sacs, the pylorie-c:cea. These may be very numerous. The intestine in all fishes except teleosts has a spiral valve in the form of a ridge running spirally along the wall and projecting into the interior of the intestine. The alimentary canal opens either with the urino-genital ducts into a common chamber, the cloaca, or, as in tele osts, ganoids, and Dipnoi, separately to the ex terior. There are no salivary glands. Sec Au D1ENTARY SYSTEM, EVOLUTION OF.
The excretory organs in all fishes are a pair of glands situated just under the backbone and protruding into the body-eavity. The excretion is carried away by a ureter, which empties va riously in the different groups of fishes. In the elasmobranehs and Dipnoi the kidneys extend for about two-thirds the length of the body-eavity, and the ureters, having united, open into the cloaca as a common duet. In the teleosts and ganoids the glands may occupy the entire length of the body-cavity. The ureters open into a uri nary bladder, and this into the urino-genital sinus, the latter opening separately to the ex terior. See EXCRETORY SYSTEM, COMPARATIVE ANATOMY OF.