the heart contains only venous blood. All the blood on its course to the system passes through the gills first and is there purified. In the lung-fishes, where the air-bladder functions as a lung, some arterial blood reaches the heart from the air-bladder by the pulmonary vein. This empties into the left side of the sinus venosus. The sinus venosus, the auricle, and the conus are imperfectly divided in the lung-fishes, suggesting the condition in amphibia. The blood-corpuscles of fishes are nucleated.
The central nervous system in fishes consists, as in other vertebrates, of a brain and spinal cord and the sympathetic system. The brain presents the usual divisions of the higher forms. It lies in the same plane with the spinal cord, and exhibits no flexures. The brain does not completely fill the cranial cavity, and the intervening space is filled with the gelatinous arachnoid tissue. In the teleosts the optic lobes and the cerebellum constitute the largest divisions, the cerebrum remaining very poorly developed. In the elasmobranchs the olfactory lobes may be enormously developed. Ten cranial and many spinal nerves leave the brain and spinal cord. The sympathetic system presents the usual character and relations found in vertebrates. The emotions of fishes (manifestations of anger, fear, etc.), indicative of the mental status, are extensively considered in the Proceedings of the Zoological Society of London for 1878.
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 restricted to the mouth-cavity. 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. Situated within longitudinal grooves or pits are sense 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 developed a more or less complicated system of grooves on the head. Many fishes have filamentous 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 papillæ on the head. The organs of smell are a pair of pits in the skin at the anterior dorsum 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 ampullæ, a sacculus, and utriculus. The otoliths are large. Various experiments point to the conclusion that the ear in fishes is merely an organ of equilibrium. The eyes have the usual structure of the vertebrate eye. The accessory organs, like the lids and lachrymal glands, are poorly developed. Eyes may be absent in cave and deep-sea forms. See Nervous System, Evolution of.
The digestive 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 indicated. The gullet and stomach vary with the food habits of the fish. Predatory fishes swallow and stow away large objects. An extreme instance is the deep-sea fish Chiasmodon 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 pyloric-cæca. 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 teleosts, ganoids, and Dipnoi, separately to the exterior. There are no salivary glands. See Alimentary System, Evolution of.
The excretory organs in all fishes are a pair of glands situated just under the backbone and protruding into the body-cavity. The excretion is carried away by a ureter, which empties variously in the different groups of fishes. In the elasmobranchs and Dipnoi the kidneys extend for about two-thirds the length of the body-cavity, and the ureters, having united, open into the cloaca as a common duct. In the teleosts and ganoids the glands may occupy the entire length of the body-cavity. The ureters open into a urinary bladder, and this into the urino-genital sinus, the latter opening separately to the exterior. See Excretory System, Comparative Anatomy of.
Reproduction. The sexes are separate. The testes and ovaries are paired organs varying in shape and position in the abdominal cavity with the different groups. The products of the male and those of the female may or may not be led to the exterior by a duct. In the male this may be a more or less convoluted vas deferens or a simple continuation of a bag. In teleosts the testes or ovaries are simply continued posteriorly as a duct which empties into the urino-genital sinus. In case there is no oviduct the eggs break free into the body-cavity and pass into the urino-genital sinus by a pair of slits in the anterior wall. In ganoids there is always an oviduct. In the elasmobranchs and Holocephali there is an oviduct, usually quite highly developed, opening into the cloaca. In many teleosts and in nearly all elasmobranchs and Holocephali the eggs are fertilized in the body of the mother. In many instances the egg develops there to quite an advanced stage. In all other fishes the milt is poured over the eggs as they are extruded, or into the water in the immediate vicinity. In elasmobranchs and Holocephali the ‘claspers’ act as intromittent organs, by which the milt is introduced into the body of the female. In the ovoviviparous teleosts the anterior portion of the anal fin is modified into an intromittent organ.
Breeding Habits. Fishes that lay eggs show no parental care, as a rule, either for their eggs or young. The eggs are fastened to rocks or weeds or other objects, and the eggs and young are left to shift for themselves. In many marine species the eggs are extruded into the water, and
