Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines. The seven extant species of sea turtles are: the green, loggerhead, Kemp's ridley, olive ridley, hawksbill, flatback, and leatherback. The majority of a sea turtle's body is protected by its shell. The turtle's shell is divided into two sections: carapace (the dorsal portion) and plastron (the ventral portion). The shell is made up of smaller plates called scutes. The leatherback is the only sea turtle that does not have a hard shell. Instead, it bears a mosaic of bony plates beneath its leathery skin. In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts. The reduced volume of a fusiform body means sea turtles can not retract their head, legs, and arms into their shells for protection like other turtles can. However this more stream-line body plan reduces drag in the water and allows the turtle to swim more easily. The leatherback is the largest species of sea turtle. Measuring 2–3 meters (6–9 ft) in length, and 1-1.5 m (3–5 ft) in width, weighing up to 700 kilograms (1500 lb). Other species are smaller, being mostly 60–120 cm (2–4 ft) and proportionally narrower.