INTERNAL STRUCTURE. The respiratory organs of fishes consist of gills. and in the case of Dipnoi, of gills and lungs. In the region of the pharynx the alimentary canal communicates with the exterior on each side by a series of slits called gill-clefts. The water passing through the mouth into the pharynx escapes to the exterior through these gill-clefts. The bars bounding these clefts have attached to them the gills, which are merely the mucous membrane of the bars raised up into a number of ridges, called branchial fila ments. These are highly vascular, the blond entering them being venous in character, and they constitute the true respiratory organs. The water passing through the clefts bathes the ments and effects the necessary interchange cf gases. In the lung-fishes the air-bladder has assumed the function of a lung. This organ is not a smooth-walled bag, as in other fishes, hut a highly vascular, much-chambered organ. The air enters it through a connection with the pharynx. See GILL; RESPIRATORY SYSTEM.
The air or swimming bladder of fishes is a sac, usually unpaired, filled with gas and lying dorsal to the intestine. Embryologically it corre sponds with the lungs, as it arises as a divertie ulum of the intestine, and in this connection may persist as the pneumatic duct, or in other cases may he wholly lost. The function of the air-blad der is not always clear. Whed it is supplied with venous blood, as in Dipnoi and Amia, and its gases arc periodically exchanged for outside it doubtless functions as a lung. When it is supplied with arterial blood, or when it is a closed sac, its function is supposed to be hydro static. It may, in addition, serve as a storehouse for oxygen taken in by the gills. The contrac tion and expansion both of the bladder and of the body musculature serve to condense and expand the air in the bladder, and thus may aid the fish in rising or sinking in the water. Unequal anterior, posterior, or lateral pressure on the bladder may likewise aid the fish in directing its course. In some fishes the forked anterior end of the air-bladder fits closely against the pos. terior wall of the auditory capsule. In carps and siluroids the bladder and auditory organs are connected by a chain of bones. Such connections doubtless enable the fish to become more keenly sensitive to any change in hydrostatic pressure in the bladder.
Except in teleosts, where a coitus arteriosus is wanting, the heart of fishes consists of (I) a sinus venosus, (2) one auricle, (3) one ven tricle, and (4) a cones arteriosus. In the teleosts the corms is represented by the bulbus arteriosus, which, however, is a part of the aorta and does not undergo rhythmical contraction like the coi's. The sinus venosus is a thin-walled expansion of the afferent veins, and a sort of antechamber to the thin-walled auricle. From
the latter the blood pusses into the thick-walled, muscular ventricle; thence either into the yen tral aorta (teleosts) or into the coitus. From the colitis the ventral aorta extends forward a short distance, and then divides on each side into a number of branches (afferent branchial arter ies), which pass through the gill-arches, break ing.: up there into capillaries in the gill-filaments, which re-collect into the efferent branchial ar teries. These unite above the pharynx as a single large artery, the dorsal aorta, which passes back ward through the entire length of the body, sup plying the different organs. The most important branches given off are the carotids to the head. the subelavian to the pectoral fins, the mesenteric and cadiac to the digestive organs, the renal to the kidneys, the spermatic or ovarian to the re productive glands, and the iliac to the tins. Posteriorly the aorta is as the ca tidal artery. From the anterior part the blood is returned by the jugular vein; from the pee fin by the sulielavian; from the digestive system by the hepatic-portal to the liver: thence by the hepatic; and from the other portions of the body by the cardinal. All these enter the sinus venosus. Thus in all except the lung-flahes 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, sonic 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 conns are im perfectly 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 com pletely fill the cranial cavity, and the interven ing 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 elasmobranehs 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 char actei and relations found in vertebrates. The emotions of fishes (manifestations of anger, fear, etc.), indicative of the mental status, are exten sively considered in the Proceedings of the Zoolog ical Society of London for 1878.