SHARK (probably from Lat. carcharus, from Gk. Kapxaplas, karcharias, sort of shark, from suipxapos, karcharos, jagged; connected with Kap Ktvos, karkinos, Skt. karkata, crab, karkara, hard). The name given to such elasmobranch fishes (see ELASMOBRANCHH) as have their gill openings lateral instead of ventral. as in the skates ( Batoidea). The body is nearly always elongate, tapering gradually to the tail and not much thickened in the middle. The muzzle pro jects over the mouth; the nostrils are situated on the under side of the muzzle. Time males have clasper.;. There are usually two dorsal fins. but in the small order of notidnnoid sharks there is only a single one. The gill-openings are five, excepting in the cow sharks where there are six or seven. The skin has no scales, but minute dentieles, much resemblirig teeth in their develop ment and structure. The teeth are generally large. sharp, and formed for cutting, with the edge often serrated. In the cfmstracionts (q.v.) they are pavement-like, and in some genera are small and numerous. • As the rows of teeth on the ridge of the jaw are worn away they are continually replaced by new series.
The teeth of sharks are dermal structures never ankylosed to the jaw or to any other skeletal part, hut are imbedded in a tough fibrous membrane and are arranged in concen tric rows. The row of dentieles that occupies the border of the jaw is erect. Adjacent rows are only partially erect, while those behind lie recumbent. The fibrous gum moves up and out ward over the surface of the jaw and carries each successive row of teeth to a functional posi tion on the jaw. When a row of teeth leas passed this point the teeth fall out. This fact accounts for the great number of shark's teeth which are preserved in geological deposits, for each shark during its life casts off a great many teeth. Both in form and structure the dermal spines on the external skin of certain sharks cannot be distinguished from the spines that oc cur in the mouth and function as teeth.
Most sharks are carnivorous and voracious, some of them taking objects as large as man. Some live on small marine organisms and a few are herbivorous. Some species are ovoviviparous; others lay eggs. The eggs are large in compari son with those of osseous fishes, and are square or oblong in form, with a tough horny coat, each corner prolonged into a tendril, apparently of use for their entanglement among seaweeds to prevent being thrown about. In some of the vivip arous species the embryo is attached to the walls of the uterus by a sort of placenta. Sharks are found in all seas, but are most abundant in the tropics. They are nearly all marine, a few (watering fresh water, and one species living con tinually in Lake Nicaragua.
The rough skin of sharks is employed by join ers for polishing fine-grained wood, and for cov ering the hilts of swords, tools, and the like. to Make them firmer iu the grasp. (See Site GREEN.) The flesh is coarse, but is sometimes eaten. The fins abound in gelatin, and are much used by the Chinese for making a rich gelatinous soup. The liver yields a large quantity of valu able oil. See OIL-SHARK.
The sharks embrace several families, among which prominent ones are the Hexanchidie (cow sharks), Cestraciontidu (Port Jackson sharks), Bet erodontithe (hull-head sharks), Ginglymosto matithe (nurse-sharks), Galeithe (dog-sharks, topes, tiger-sharks, man-eaters, requiems. etc.), Sphyrinithe (hammer-heads), Alopiidfe (thresh ers), Carchariid:e (sand-sharks), Lamnidn (or beagles), Cetorhinid:e (the basking-sharks), and Squalidai (dog-fishes). Most of these will be found described under their common names. Consult the authorities mentioned under Fisll. See Plates of GREAT SHARKS; LAMPREYS AND DOGFISH; PHILIPPINE FISHES.