SHARK

any of numerous species of cartilaginous fishes of predatory habit that comprise the order Selachii (class Chondrichthyes). Sharks, together with rays and skates, comprise the subclass Elasmobranchii of the Chondrichthyes. Sharks differ from other elasmobranchs, however, and resemble ordinary fishes, in the fusiform shape of their body and in the location of their gill clefts on each side of the head. Though there are exceptions, sharks typically have a tough skin that is dull gray in colour and is roughened by toothlike scales. They also usually have a muscular, asymmetrical, upturned tail; pointed fins; and a pointed snout extending forward and over a crescentic mouth set with sharp triangular teeth. Sharks have no swim bladder and must swim perpetually to keep from sinking to the bottom. There are more than 300 living species of sharks, taxonomically grouped into 13 to 22 families according to different authorities. Several larger kinds can be dangerous to humans; smaller ones, called topes, hounds, and dogfishes, are fished commercially.