Encyclopædia Britannica, Ninth Edition/Tortoise

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TORTOISE.Of the three names generally used for this order of reptiles, viz., Tortoise, Turtle, and Terrapin, the first is derived from the old French word tortis, i.e., twisted, and was probably applied first to the common European species on account of its curiously bent fore legs. Turtle is believed to be a corruption of the same word, but the origin of the name terrapin is unknown: since the time of the navigators of the 16th century it has been in general use for freshwater species of the tropics, and especially for those of the New World. The name tortoise is now generally applied to the terrestrial members of this group of animals, and that of turtle to those which live in the sea or pass a great part of their existence in fresh water.

Tortoises and turtles constitute one of the orders of Reptiles, the Chelonia. They are characterized by having the trunk of the body incased in a more or less ossified carapace, which consists of a dorsal more or less convex portion, and of a flat ventral one, the so-called plastron. These portions are generally more or less firmly united on the side, but leave a wide opening in front through which the head and neck and the fore-limbs protrude, and one behind for the tail and hind-limbs. The dorsal carapace is (with the exception of Sphargis) formed by the dorsal vertebrae, by the ribs which are so much expanded as to form sutures with each other, and by a number of lateral dermal ossifications (marginals). The plastron consists of from eight to eleven more or less dilated dermal bones, the sternal elements of higher Vertebrata being absent. This osseous case or shell receives in its interior the organs of the chest and abdomen, the humeral and pelvic bones, and the muscles for the humerus and femur. In many species, especially those of the family Tesludinidx, or tortoises proper, the neck and head and the limbs can be withdrawn within the shell, the cervical and the proximal caudal vertebrae retaining their mobility. In the majority of Chelonians the osseous shell is covered with a hard epidermoid coat, which is divided into large symmetrical plates (commonly called " tortoise-shell " in those species from which the article of commerce is obtained), which can be detached from the underlying bones. These epidermoid plates do not correspond in arrangement or extent with the bones of the carapace; they vary considerably in form, and are therefore generally noticed in the descriptions of species. Their arrangement and terminology may be learned from the accompanying illustrations (figs. 1, 2).

The integuments of the head, neck, tail, and limbs are either soft and smooth or tubercular or scaly, the tubercles and scales having frequently an osseous nucleus.

Other parts also of the skeleton show remarkable peculiarities, so that the sometimes very fragmentary remains of Chelonians can almost always be recognized as such. All the bones of the skull are suturally united, with the exception of the mandible and hyoid bone; the dentary portion of the mandible consists of one bone only. The pectoral arch is composed of the scapula, with which the precoracoid is united, and the coracoid. Clavicles (epiplastra) are represented by the anterior elements of the plastron. Two pairs of limbs are invariably present.

All Chelonians possess a tail, which is generally short, but sometimes elongate, and always provided with strong muscles at the base. No Chelonian possesses teeth; but their jaws are provided with horny sheaths, with hard and sharp edges, forming a beak like that of a parrot.

The number of Chelonians known at present may be estimated at about 220, the freshwater species being far the most numerous, and abundant in well- watered districts of the tropical and subtropical zones. Their number and variety decrease beyond the tropics, and in the north they disappear entirely about the 50th parallel in the western and about the 56th in the eastern hemisphere, whilst in the southern hemisphere the terrestrial forms seem to advance to 36 S. lat. only. The marine turtles, which are spread over the whole of the equatorial and subtropical seas, sometimes stray beyond those limits. As in other Page:Encyclopædia Britannica, Ninth Edition, v. 23.djvu/476 Page:Encyclopædia Britannica, Ninth Edition, v. 23.djvu/477 Page:Encyclopædia Britannica, Ninth Edition, v. 23.djvu/478 Ocean (fig. 4). It is carnivorous, feeding on fish, molluscs, and crustaceans, and not esteemed as food, although it is eaten by the native fishermen. A great part of the turtle-oil which finds its way into the market is obtained from the Atlantic species of this genus; also tortoiseshell of an inferior quality is obtained from it.

FIG. 7. The Matamata (Chelys fimbriata), with side view of head, and separate view of plastron.

The Green Turtle (fig. 5), which yields the mate rials for the celebrated soup, belongs to the genus Chelonia; it is dis tinguished from Caouana by having thirteen ver tebral and costal shields only, which are not im bricate. These animals are herbivorous, feeding on marine Algx only; they occur in the IndoPacificand Atlantic; and, although several species have been distinguished, they all may possibly be referable to one only.

Fig. 8. Upper View of the Turtle of the Euphrates (Trionyx euphratica).

The turtle imported into Europe comes chiefly from the West Indies. Instances are recorded of the flesh of this species having acquired poisonous qualities. The Hawksbill Turtle, Caretta (fig. 6), so named from its rather elongate and compressed curved upper jaw, does not reach the same size as the other turtles, and is readily recognized by the thirteen imbricate scutes of its carapace. It seems to be more abundant in the Indian than in the Atlantic Ocean, but is plentiful only in certain localities. As, however, these turtles always re sort to the locality where they were born, or where they have been wont to propagate their kind, and as their capture is very profitable, they become scarcer and scarcer at places where they are known to have been abundant formerly.

FIG. 9. Lower View of Trionyjc euphiatica.

If the plates of tortoiseshell are detached from the animal when decom position has set in, their colour becomes clouded and milky, and hence the cruel expedient is resorted to of suspending the turtle over fire till heat makes the shields start from the bony part of the cara pace, after which the creature is permitted to escape to the water. There is no doubt that turtles thus allowed to escape to the water after such an operation may survive; but it is very improbable that the epidermal shields are ever sufficiently regenerated to be fit for use. At Celebes, whence the finest tortoiseshell is exported to China, the natives kill the turtle by blows on the head, and immerse the shell in boiling water to detach the plates; dry heat is only resorted to by the unskilful. The natives eat the flesh of this turtle, but it is unpalatable to Europeans; the eggs, however, are regarded as equal to those of the other turtles.

Of the family Chelydidse the most remarkable type is the Matamata, Chelys fitnbriata, a native of the Guianas and northern Brazil (fig. 7). In its strongly depressed and flat head, long tubelike snout, weak jaws, minute eyes, skinny tentacles, it bears a striking similarity to the Surinam toad, Pipa americana, which inhabits the same countries. The neck is very broad and depressed, and fringed with foliated tentacles, floating in the water like some vegetable growth, whilst the rough bossed carapace resembles a stone, an appearance which evidently is of as great use to this creature in escaping the observation of its enemies as in alluring to it unsuspicious animals on which it feeds.

The family of Carettochelydidx contains a single genus, Carettochelys, quite recently discovered in the Fly river, New Guinea, and exhibiting a remarkable combination of characters. Its limbs are formed very much like those of the marine turtles, whilst the shell lacks epidermic scutes, as in the Trionychidx.

In the freshwater turtles, or Trionychidae (figs. 8 and 9), the cara pace is reduced to a flat disk, which is covered with soft skin. The neck and limbs can be lodged under the broad skinny borders of the carapace; also the plastron is very imperfectly ossified, and some times dilated into large flexible lobes which may cover the limbs. The latter are much flattened and broadly webbed, and only the three inner toes armed with claws. The jaws are concealed under broad, fleshy lips, the nose projecting like a short proboscis. These turtles are carnivorous, and very ferocious; when they want to bite or seize their prey they project their neck and head with lightning rapidity. They are well known on the upper Nile, Euphrates, Ganges, Yangtse-kiang, and Mississippi, and, indeed, distributed over all the large fresh waters of the geographical regions to which these rivers belong. Some of the species exceed a length of 3 feet. In the United States, where twospecies, Trionyx muticus and Trionyx ferox, occur, the flesh of the latter is said to be most delicate to eat, far surpassing in flavour that of the green turtle. (A. C. G.)