the notochord, which in the teleosts persists only as remains in the cavities between the adjoining centra, is a continuous rod. The neural and hæmal arches and the ribs are variously developed. In most of the elasmobranchs there are present definite biconcave vertebræ with neural and hæmal arches, transverse processes, and rudimentary ribs, but they remain cartilaginous or become only slightly ossified. The centra are pierced by a canal through which remains of the notochord are continuous. The posterior end of the spinal column forms the basis of the caudal fin. See Skeleton.
VERTEBRÆ OF A CARTILAGINOUS FISH.
| 1. Side view. 2. Longitudinal section. 3. Transverse section of caudal vertebra of a shark. a, Centrum; b, neurapophysis: c, intercrural cartilage; d, hæmapophysis; e, spinal canal; f, intervertebral cavity; g, central canal of persistent portion of notochord; h, hæmal canals for blood-vessels. |
The fins are supported by cartilaginous or
bony rods. In the dorsal and anal these rods do
not join those of the internal skeleton directly,
but, imbedded in the flesh, are interposed between
the spinous processes of the vertebræ. The
paired fins, not always all present, represent the
typical fore and hind limbs of quadrupeds. They
consist of a basal set of bones, varying in number
and arrangement in the different groups, bearing
the radiating fin-rays, and articulating proximally
with the pelvic or shoulder girdle. The
girdles are cartilaginous in elasmobranchs,
lung-fishes, and sturgeons; but in teleosts this, like the
chondrocranium, has additions in the way of
dermal bones, which have become associated with
it. The pelvic bones may be imbedded in the
muscles of the abdomen, or may occur farther
anterior and become fastened to the pectoral
girdle. In some fishes the pelvic fins, which
answer to the hind feet of quadrupeds, are
actually farther forward than the pectoral fins,
and are then called jugular. In some fishes, as
in the common eel, the ventral fins are wanting,
while in others both pairs may be absent. In
lung-fishes “the skeleton of the pectoral fin
consists of a stout basal cartilage, an elongated,
tapering central axis made up of a number of
short cartilaginous segments, and two rows of
jointed cartilaginous rays extending out on either
side of the axis.” See Fin.
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 filaments. These are highly vascular, the blood entering them being venous in character, and they constitute the true respiratory organs. The water passing through the clefts bathes the filaments and effects the necessary interchange of 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, but 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 corresponds with the lungs, as it arises as a diverticulum of the intestine, and in this connection may persist as the pneumatic duct, or in other cases may be wholly lost. The function of the air-bladder is not always clear. When it is supplied with venous blood, as in Dipnoi and Amia, and its gases are periodically exchanged for outside air, 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 hydrostatic. It may, in addition, serve as a storehouse for oxygen taken in by the gills. The contraction 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 posterior wall of the auditory capsule. In carps and siluroids the bladder and auditory organs are connected by a chain of bones. Such connection) doubtless enable the fish to become more keenly sensitive to any change in hydrostatic pressure in the bladder.
Except in teleosts, where a conus arteriosus is wanting, the heart of fishes consists of (1) a sinus venosus, (2) one auricle, (3) one ventricle, and (4) a conus arteriosus. In the teleosts the conus is represented by the bulbus arteriosus, which, however, is a part of the aorta and does not undergo rhythmical contraction like the conus. 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 passes into the thick-walled, muscular ventricle; thence either into the ventral aorta (teleosts) or into the conus. From the conus the ventral aorta extends forward a short distance, and then divides on each side into a number of branches (afferent branchial arteries), which pass through the gill-arches, breaking up there into capillaries in the gill-filaments which re-collect into the efferent branchial arteries. These unite above the pharynx as a single large artery, the dorsal aorta, which passes backward through the entire length of the body, supplying the different organs. The most important branches given off are the carotids to the head, the subclavian to the pectoral fins, the mesenteric and cœliac to the digestive organs, the renal to the kidneys, the spermatic or ovarian to the reproductive glands, and the iliac to the pelvic fins. Posteriorly the aorta is continued as the caudal artery. From the anterior part the blood is returned by the jugular vein; from the pectoral fin by the subclavian; 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-fishes
