Sharks can be found in all the world's seas, from the equator to the polar regions, in shallow as well as deep waters. We are currently aware of about 480 species of sharks, occurring in a variety of shapes and sizes. Some species, labeled benthic, dwell mainly on the seafloor. Others, known as pelagic, spend much of their time navigating the open seas, although they may also be found on the ocean bottom. Some species inhabit rivers and lakes. One important distinguishing feature of sharks is that their skeletons are made of cartilage, while those of most other types of fish are made of bone. Other cartilaginous marine animals include rays, skates, and chimaeras, and they--as well as sharks--are grouped in the class Chondrichthyes. Bony fishes, on the other hand, are placed in the class Osteichthyes. Generally speaking, a shark has a streamlined body with a long, flattened snout, a parabolic mouth on the ventral (lower) side, and eight fins: two pectoral and two pelvic fins, and one each of the first dorsal, second dorsal, anal, and caudal (tail) fins. The upper lobe of the caudal fin is noticeably longer than the lower one. Furthermore, the male's pelvic fins are extended to form a pair of organs called claspers, which are used to impregnate females. The body shape, however, varies according to the way of life of the species. For example, fast-swimming mackerel sharks have a large, conico-cylindrical shape; benthic cat sharks have long, slender bodies; and benthic angel sharks and saw sharks are highly flattened in shape. Hammerhead sharks have unique, rectangular heads
