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a subclass of reptiles. Testudinates are characterized by a body covered with a shell consisting of a convex dorsal portion (carapace) and a flat ventral portion (plastron). The two portions are rigidly connected on either side by a bridge of bone or freely connected by skin. The shell consists of dermal ossifications, as well as of enlarged ribs and riblike processes of the vertebrae. The clavicles and sternum are part of the plastron. The bony elements of the shell are called plates. In most testudinates they are covered with horny scutes and only in soft-shelled turtles and in the leatherback are they covered with skin.
In land tortoises the carapace is highly arched (rounded or parabolic in cross section) and can bear substantial loads, either evenly distributed or concentrated and percussive. In freshwater and marine species, it is flattened (elliptical) and “streamlined.” The plastron in these testudinates is partially reduced or is segmented into mutually mobile parts, which increases the freedom of movement of the legs, head, and tail. The flattening and lightening of the shell is often compensated by the proliferation of nodular protrusions and “stiffening ribs” on the surface of the carapace, which strengthen it. The cranium is of the armored type (pseudostegalian structure) in living marine species; in the rest of the testudinates it has a false temporal arch (pseudapsidal type).
The testudinates have no teeth; the jaws are covered with horny plates, which form a semblance of a bill. The unique structure of the cervical vertebrae, which differ in shape, ensures great mobility of the neck. When a testudinate hides its head under the shell, the neck retracts either in a vertical plane (S-necked type) or in a horizontal plane, that is, sideways (side-necked type). There are usually few caudal vertebrae (the tail is somewhat longer in freshwater species). Development of the shell has resulted in the transfer of the girdles of the extremities beneath the ribs, an exceptional occurrence in vertebrates. In freshwater species the extremities are more mobile and have webbed digits. In sea turtles the legs have been modified to flippers, with the anterior ones being longer than the posterior ones.
The presence of a shell has considerably affected the internal structure of testudinates. The musculature of the trunk is partially reduced, while on the extremities and in the neck region it is considerably developed. Owing to the absence of costal respiration, air is “swallowed” by means of the sublingual apparatus, partially owing to movements of the extremities. In freshwater species gas exchange is partially effected by means of special anal sacs opening into the cloaca. Soft-shelled turtles have cutaneous respiration, a rare phenomenon in reptiles. The level of basal metabolism is low, and the intensity of metabolism is weak. In excretion, urea is principally discharged, and consequently the urinary bladder is large (in most other reptiles uric acid is primarily excreted; the urinary bladder is absent in many). The brain and the principal receptors are poorly developed.
Testudinates vary considerably in size. The largest living representatives are the sea turtles (the leatherback is up to 2 m long). In more ancient times, testudinates attained a length of 3 m.
Testudinates are descended from one of the branches of the oldest terrestrial reptiles—Cotylosauria. In the course of evolution, many became adapted to life in continental waters and seas. Testudinates are reliably known from the Triassic, although their appearance as early as the end of the Paleozoic cannot be excluded. They flourished from the second half of the Mesozoic to the end of the Neogene. Many genera and families were distributed more widely in past geological eras than today. For example, soft-shelled turtles, side-necked turtles, snapping turtles, and elephant tortoises once inhabited Europe; today they are preserved only on other continents. The majority of living testudinates inhabit tropical and equatorial latitudes.
Living testudinates are classified in five orders: Cryptodira, Pleurodira, Trionychoidea, Chelonioidea, and Dermochelidae. The order Dermochelidae has one species, the leatherback, which is sometimes classified in the order Chelonioidea. There are more than 210 species of living testudinates; terrestrial and freshwater species are found on all continents except Antarctica and on many large islands. There are seven species in the USSR: the European pond tortoise (Emys orbicularis), Mauremys caspica, the spur-thighed tortoise (Testudo graeca), Agrionemys horsfieldi, and the spiny soft-shelled turtle (Trionyx sinensis), which are land tortoises, and the loggerhead (Caretta caretta) and the leatherback (Dermochelys coriacea), which are sea turtles.
Some testudinates live only on land (43 species), where some have adapted to digging burrows. Sea turtles, of which there are five species, live in the ocean, except during the reproductive period. The majority of living testudinates are semiaquatic, inhabiting rivers, lakes, marshes, and desalinated bays. During the cold or dry seasons, testudinates may fall into a torpor (winter and summer hibernation), enabling them to survive unfavorable weather conditions. Terrestrial species are basically herbivorous, while marine species are primarily so. Freshwater species feed mainly on other animals, such as fish, amphibians, and invertebrates. Testudinates can do without food for several months at a time. They usually mate on land, where they also lay their eggs (one to several hundred), which are buried in the ground. Some species lay eggs up to three or more times a year. The spherical or elliptical eggs are covered in most species with a limestone shell; in marine and some freshwater species they are covered with a leathery shell. The incubation period is two to three months. Sexual maturity is attained not earlier than two to three years of age. The males are usually smaller than the females; they have a slightly concave plastron, which may be a little higher than the carapace. Growth in testudinates is unlimited, slowing down considerably with the onset of sexual maturity. The life-span is several decades, with some species living more than 100 years.
A number of species are hunted; these are mainly sea turtles and a few freshwater species, more rarely terrestrial species. The flesh, fat, eggs, and sometimes the horny scutes are used, the last for various clothing accessories. There are farms for breeding and fattening turtles. Some freshwater species cause minor damage to the fishing industry; land tortoises damage agricultural crops. Many species are protected; in the USSR all land-tortoise species and the freshwater spiny soft-shelled turtle are protected. (SeeCRYPTODIRA; PLEURODIRA; TRIONYCHOIDEA; CHELONIOIDEA; and LEATHERBACK.)
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L. I. KHOZATSKII