TORTOISE, the name applied to some members of the order CHELONIA, a group of REPTILES (q.v.) ; the name is not con sistently applied to any one section of the order but is generally used to designate a terrestrial animal, "turtle" being in Britain the equivalent name for a marine species (though in America it is more generally used), and "terrapin" or "water-tortoise" for a fresh-water chelonian. They are toothless reptiles with well developed limbs and a "shell" covering the body.
This characteristic "shell" consists of an upper carapace and a lower plastron, the two being more or less firmly united at the sides by the so-called bridge. Both carapace and plastron proper are bony structures built up from a number of elements which are firmly united by sutures; the carapace con sists of a central row of from 10-12 plates most of which are fused to the vertebrae, a lateral row of 8 plates on each side fused to the ribs and about 23 small plates bordering its free edge ; the plastron contains five paired elements and one unpaired. The union of the upper and lower halves of the shell may be rigid and bony or cartilaginous and flexible and in some forms trans verse hinges are also developed. In the majority of chelonians the bony shell is overlaid by horny plates, but these do not cor respond with the underlying bones although their general arrange ment is similar; they are of exactly the same nature as the horny scales which cover the head, limbs and exposed soft parts.
The shape of the "shell" varies in conformity with the habits of the animals in strictly terrestrial forms it is usually high, domed, and sufficiently large to permit the head and limbs to be withdrawn into it, whereas in aquatic species, it is generally de pressed and so much reduced in size that it offers little protection for the limbs. Correlated also with the habitat is the shape of the limbs; in terrestrial species they are club-shaped, all the digits being bound together and only externally distinguishable by their claws whereas in the fresh-water dwellers the digits remain dis tinct and are connected by a web; the marine turtles have the fingers and toes all bound together and the number of claws reduced, but the whole limb is here transformed into a paddle.
As a result of the development of the shell and the consequent fixation of the ribs the method of breathing is rather different in the chelonia as compared with other reptiles and resembles that of the amphibia; air is pumped into the lungs by movements of the hyoid apparatus which alternately distends and compresses the neck and floor of the mouth and this action is assisted to some extent by the piston-like movements of the head and limbs in and out of the shell when the creature is active. Additional respiratory mechanisms have been developed by some aquatic species by means of which the oxygen dissolved in the water can be utilised.
No recent chelonian possesses teeth, the jaws being covered with horny beak-like plates which usually have sharp cutting edges; terrestrial species are, as a rule, strictly herbivorous and though many aquatic species are impartial in their feeding habits the majority are carnivorous, many raptorial. The voice is not well developed, females as a rule only being able to hiss; males, however, in the breeding season at least utter cries which in smaller species may be only a feeble piping but in the Giant Tortoises are hoarse bellows. Reproduction is always by means of eggs which are spherical and covered by a parchment-like or calcareous shell; they are buried in sand or loose earth.
There are living at the present day some 225 species but these are probably only the surviving remnant of a formerly much more numerous group. They are cosmopolitan in their dis tribution except for the more recent islands and those northern and southern countries where the winter is too severe ; in England their extermination seems to have been relatively recent.