The "hidden-neck" turtles (Cryptodira) retract their head with the neck in a vertical S-shape, appearing to pull the head directly into the shell with nothing showing but the snout. This is a larger and much more diversified group, and includes the pond turtles, the land-dwelling tortoises, and the large sea turtles. Members of this group are found throughout the temperate and tropical regions of the world.
Three of the more primitive families are found mainly in North America, with some ranging also into Central and South America. The snapping turtles comprise two genera, each with a single species. The common snapper (Chelydra serpentina) is best known, and ranges from Canada to Ecuador. It is a large turtle, with a shell length up to 18 in (47 cm), and a long tail. The even larger alligator snapper (Macrochelys temminckii) can exceed 24 in (66 cm) in shell length. It feeds mainly on fish that it attracts with a worm-like "bait" on its tongue. Both species lay 20-80 spherical eggs in a flask-shaped hole dug into a sandy bank.
The mud and musk turtles include about 25 small- to medium-sized species found from Canada to Brazil. Most of these are carnivorous, feeding on insects, worms, and other small animals. They lay only a few (two to 10) elongate, brittle-shelled eggs.
The highly aquatic Central American river turtle (Dermatemys mawii) is a large species, with a shell length of up to 25 in (65 cm). It is seldom found more than a few feet from water, is herbivorous, and highly prized as food by people living in its range.
The pond turtles (Emydidae) and tortoises (Testudinidae) comprise the largest numbers of turtle species, and occur throughout the tropical and temperate regions of the world, other than Australasia. The American pond turtles are closely related to the pond turtles of southern Asia, as are the European turtles.
The pelagic sea-turtles comprise only a few genera. They are extremely large, conspicuous animals, and once occurred in huge numbers. Sea turtles spend almost all of their lives in the open sea, but must come to land to lay their eggs. They nest on certain tropical and subtropical beaches, and return year-after-year to these same places. Once the nesting beaches were in remote locations, but no longer; Miami Beach, for example, was once an important nesting place.
The largest of the sea-turtles (and the biggest living turtle) is the giant leatherback. It lacks the horny plates that cover the shells of most turtles, and instead has a smooth leathery covering. It largely feeds on jellyfish and is the fastest swimmer of all turtles. It has a thick fatty layer under the skin that helps to retain body heat, as well as a heat-exchanging circulatory mechanism that conserves heat generated by muscular effort. These allow this species to range into cool waters during the northern summer, when jellyfish are abundant there.
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