Online Encyclopedia

TORTOISE

Online Encyclopedia
Originally appearing in Volume V27, Page 67 of the 1911 Encyclopedia Britannica.
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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 forelegs. 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 fresh-water 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. They constitute one of the orders of reptiles, the Chelonia: toothless reptiles, with well developed limbs, with a dorsal and a ventral shell composed of numerous bony plates, large firmly fixed quadrates, a longitudinal anal opening and an unpaired copulatory organ. The whole shell consists of the dorsal, more or less convex carapace and the ventral plastron, both portions being joined laterally by the so-called bridge. The carapace is (with the exception of Sphargis) formed by dermal ossifications which are arranged in regular series, viz. a median row (1 nuchal, mostly 8 neurals and 1-3 supracaudal or pygal plates), a right and left row of costal plates which surround and partly replace the ribs, and a consider-able number (about 11 pairs) of marginal plates. The plastron consists of usually 9, rarely II, dermal bones, viz. paired epi-, hyo-, hypo- and xiphi-plastral plates and the unpaired endo-plastral; the latter is homologous with the interclavicle, the epi-plastra with the clavicles, the rest with so-called abdominal ribs of other reptiles. In most Chelonians the bony shell is covered with a hard epidermal coat, which is divided into large shields, commonly called " tortoiseshell." These horny shields or scutes do not correspond in numbers and extent with the underlying bones, although there is a general, vague resemblance in their arrangement; for instance, there is a neural, a paired costal and a paired marginal series. The terminology may be learned from the accompanying illustrations (figs. 1 and 2). The integuments of the head, neck, tail and limbs are either soft and smooth or scaly or tubercular, frequently with small osseous nuclei. All the bones of the skull are suturally united. The dentary portion of the mandible consists of one piece only, both halves being completely fused together. s The pectoral arch remains separate in the median line; it consists of the coracoids, which slope backwards, and the scapulae, which stand upright and often abut against the inside of the first pair of costal plates. Near the glenoid cavity for the humerus arises from the scapula a long process which is directed transversely towards its fellow; it represents the acromial process of other vertebrates, although so much enlarged, and is neither the precoracoid, nor the clavicle, as stated by the thought-less. The tail is still best developed in the Chelydridae, shortest in the Trionychoidea. Since it contains the large copulatory organ, it is less reduced in the males. No Chelonians possess the slightest traces of teeth, but their jaws are provided with horny sheaths, with hard and sharp edges, forming a beak. The number of Chelonians known at present may be estimated at about 200, the fresh-water species being far the most numerous, and are abundant in well-watered districts of the tropical and sub-tropical zones. Their number and variety decrease beyond the tropics, and in the north they disappear entirely about the 5oth 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. only. The marine turtles, which are spread over the whole of the equatorial and sub-tropical seas, sometimes stray beyond those limits. As in other orders ,,,
End of Article: TORTOISE
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