Shark

Shark, any of about 250 species of cartilaginous fishes of marine and fresh waters, order Selachii. Sharks, with the related rays and chimeras, have a skeleton formed entirely of cartilage rather than bone. Other distinguishing features are that the gills open externally through a series of gill slits, rather than through a single operculum, and reproduction is by internal fertilization, unlike that of bony fishes. The body is fusiform, and the upper lobe of the tail is usually better developed than the lower lobe. Sharks swim by sinuous movements of the whole body; there is no swim bladder, so they must swim constantly to avoid sinking. All are extremely fast swimmers and active predators. Despite a universal reputation for unprovoked attack, only 27 out of the 250 known species have been definitely implicated in attacks on humans.