SKELETON. The skeleton of fishes consists of the skull with its visceral skeleton; the vertebral axis with its processes; the pelvic and pectoral girdles; and the supporting elements of the various fins. This in the lower cartilaginous fishes consists only of cartilage, no true bones being present. The skull, which in the higher fishes is a eomplieated structure, in the elasmo branelis consists of a rather simple cartilaginous hollow case, the ehondrocranium, inclosing the brain, and not composed of distinct pieces. As one ascends the scale, bones are added to this chondroeranium from the outside, arising as dermal ossifications; these are probably merely highly modified dermal plates. In the ganoids the ehondroeranium generally persists with cen tres of ossification present, and the whole head is incased in dermal bones. In the higher bony fishes the ehondrocranium is usually replaced by cartilage bones with many dermal bones added. To the lower part of the skull in all fishes a series of arches are attached. These form the lower jaw and the hyoid and gill arches. The backbone generally consists of a series of ver tebra which, with the exception of Lepidosteus, are biconcave. Dorsally they bear neural arches inclosing the spinal cord, and these are prolonged dorsally as a neural spine, varying in length.
Ventrally, the vertebra bear ribs in the anterior portion, and in the eaudal region there are ha-mal arches inclosing the caudal artery and vein. These arches are prolonged ventrally as a Inemal spine. In some elasmobranchs. in the ehnmeras, in the lung-fishes, and in some ganoids, there are no such definite vertebrae developed, but the notochord, which in the teleosts persists only as remains in the cavities between the adjoining centre, is a continuous rod. The neural and hmmal arches and the ribs are variously developed. In most of the elasmobranchs there are present definite biconcave with neural and hminal arches, transverse processes, and rudimentary ribs, but they remain cartila ginous or become only slightly ossified. The
eentra 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.
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 epinous processes of the 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 proxi mally with the pelvic or shoulder girdle. The girdles are cartilaginous in elasmobranehs, 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 arc wanting, while in others both pairs may be absent. In lung-fishes "the skeleton of the pectoral fin con sists 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.