Chelonia

The Chelonia include tortoises, turtles and terrapins. This group is very primitive and its geological history goes back to Triassic period. The group has been very specialized from the very beginning and has retained some of the features of the earliest anapsid reptiles. The group must have been widely distributed and in the present time the Chelonians are mainly natives of tropical and temperate zones. They live principally on vegetables but many of them also eat molluscs, crustaceans and fishes although some are exclusively carnivorous. They live a long life, sometimes for about 200 years. Like most other reptiles, the land and fresh water tortoises living in colder regions hibernate in winter as they readily succumb to the effects of cold. In warmer latitudes, they sometimes pass a similar period of quiscence in the dry season. The tortoises may be terrestrial, such as Testudo of south Europe and islands of Indian Ocean, which is herbivorous; or fresh water, terrapins, such as Chrysemys of America and Emys of Europe and Asia, which are both carnivorous. The marine Chelonia are usually known as turtles and are often very large. Chelone mydas commonly known as green or edible turtle, is over three feet long, while Sphargis, a leathery turtle without shell is over six feet long.

The body is short and broad, enclosed in a box like hard shell, consisting of a dorsal part or carapace and a ventral part or plastron. These are in most cases firmly united, apertures being left between them for the head, neck, tail and limbs. Neck is long and mobile. In Thecophora, neck bending may be in the form of a sigmoid curve in a vertical plane, or neck bending is lateral. Tail is short. Limbs are also short but fully developed. In land and freshwater tortoises they have five digits terminating in curved horny claws. In turtles the digits are closely united together forming a flipper or swimming paddle. Cloacal aperture is longitudinal.

Carapace is the dorsal part of the shell beneath which the head and tail can often be retracted. Except for slight variations in different forms, the carapace consists of a median row of usually eight neural plates which are fused with the spinous processes of eight of the thoracic vertebrae, usually from second to ninth; a nuchal plate which is a large plate in front of the first neural plate. It lies over, and is joined by ligament to, the neural spine of the last cervical vertebra; a pygal plate (sometimes two or three in number) lying behind the eighth neural plate. When three in number the first two are united together and the last to the hinder marginal; costal plates are eight in number and joined with one another by sutures. These are broad and transversely arranged and join internally with the neurals. The ribs in this region are fused with these plates most of their length but their outer ends project beyond these plates ending in marginal; marginal plates are usually eleven pairs and a median posterior one. First marginal on each side is attached to the side of the nuchal, and the last marginal in the middle is attached to the last pygal.

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