In this article we will discuss about the representative types of turtles with the help of suitable diagrams.
The term turtle usually includes the aquatic and semiaquatic forms, tortoise includes the land forms and terrapin is used for some edible freshwater species. The characteristic of chelonian organisation is the shortening and broadening of the body, together with the development of bony plates, forming a box into which the head and limbs can be withdrawn.
The total number of segments is only about 8 in the neck, 10 in the trunk and a series of reduced caudals. The shell usually includes a dorsal carapace and ventral plastron. Each of these is made up of inner plates of bone, covered by separate outer plates of horny material. The carapace includes five rows of bony plates, namely, median neurals, and paired costals and marginals.
These plates are ossifications in the dermis, attached to the vertebrae and ribs. The plastron is developed from the expanded dermal bones of the pectoral girdle together with dermal ossifications. The whole is covered in most chelonians by rows of special smooth epidermal plates forming the “tortoise” shell. A new and larger layer is added to each of these plates each year, thus, making a number of “growth rings” from which age of the tortoise can be calculated, though the outer layers often become rubbed off.
The limb girdles lie inside the encircling ribs. The pectoral girdle’s scapula meets the carapace dorsally and carries a long acromial process and a backwardly directed coracoid, both being attached by ligaments to the plastron. The ilia are attached to two sacral vertebrae. Limbs are stout and with five digits. In marine turtles they are transformed into paddles.
In recent Chelonia teeth are totally absent, both in the herbivorous and carnivorous forms. The edges of the jaws form sharp ridges, covered with horny beak. Skull is completely roofed in Chelonia, dermal bones are widely separated from the brain case. In other groups of Chelonia, the dermal roofing has been reduced or emarginated.
The heart has a muscular sinus, separated auricles, a partly divided ventricle and two equal aortic arches. Lungs are spongy attached to the dorsal surface of the shell, sometimes enclosed in a separate pleural cavity (Testudo).
Some aquatic forms (Emys) also respire by taking water into special vascularised diverticuli of urodaeum. The metabolism of tortoises is low and they can remain for long periods without breathing. For this purpose they obtain energy by anaerobic glycolysis. Further, the tissues including the brain are unusually tolerant of anoxia. In temperate climates all species hibernate regularly.
The kidney is metanephric and the nitrogenous waste vary between urea and uric acid. Cloaca is typically subdivided and reabsorption of water occurs in cloaca to form a solid whitish excretory product. In male the copulatory organ is single. A female may lay 10 clutches of 100 eggs in a year Turtles all come ashore to breed.
The marine Chelonia mydas (green turtle) make migrations of more than 1500 km, perhaps by sun compass. They place their eggs carefully in holes made by scooping away the sand with the hind feet and then the traces are covered. Geochelone gigas, the giant tortoise of the Aldabra Islands, also digs a nest for its eggs.
Chelonia mydas is also found in Bay of Bengal and is about 1 metre long. Dermochelys, the leathery turtle having no horny shell, is over 2 metre long and weighs over 500 kg.
Chelonian history starts back from the Triassic. Proganochelys (Triassochelys) was an early turtle, with a shell like that of modern forms, but still possessing teeth on the palate. The head, tail and limbs could not be withdrawn into the shell and were protected by spines. In later evolution Pleurodira, retraction of head became possible. In side-neck turtle, the neck is folded sideways.
This group was world-wide in Cretaceous and survived today in Tropical Africa (Chelus) South America and Australia. In Cryptodira, the neck is curved in a dorso- ventral plane. This type is also known from Jurassic and Cretaceous and includes most of the modern types, Dermochelys, the leathery turtle, has a carapace consisting only of a mosaic of small bony plates beneath its leathery skin. Trionyx is a freshwater turtle with a soft shell and no horny plates. Chelonia includes more than 200 species which are quite varied and widespread.
These include terrestrial animals, such as Testudo graeca and T. imarginata tortoises of Southern Europe, the freshwater tortoises such as Chrysemys and other American terrapins; and Emys, the European water tortoise. Trionyx gangeticus is found in rivers of North India and T. laithi in South India. Chelydra serpentina (snapping turtles) and Macrolemys (alligator turtles) are carnivorous feeding on fish, frogs, water-fowls, etc. Chrysemys picta (painted turtle or terrapin) feeds on insects.
Malaclemys (diamond-back terrapin) is used as food by man. Kachuga found in Indian rivers have carapace with spinous plates. Chelodina longicollis (side-necked turtle of Australia) has a S-shaped folded neck. Chelus (mata-mata) of America and Africa is a curious side-necked freshwater turtle. Testudo is a common land tortoise. Testudo abingdoni (giant land tortoise) of Galapagos islands and Indian Ocean islands weighs over 100 kg and its age varies from 200 to 400 years.
Chelone mydas (Fig. 24.2) is a marine form commonly called green turtle. It is edible and is found in Indian, Pacific and Atlantic Oceans. Forelimbs form wing-like paddles with only a single claw found on the first digit of the paddle, digits are not visible externally, hindlimbs are clawed. Head and neck are not completely retractile, jaws have denticulate edges.
Skull and shell are light and porous. Tail is very short. Carapace is flat and heart-shaped. Plastron is joined to carapace only by ligaments, there being no bony ridges, osteoderms of plastron leave a large unossified gap in the middle. Shell has osteoderms and horny scales. Marine turtles do not drink water but extract salt from the sea by means of a pair of glands situated above the eyes.
It lives mostly on seaweeds but also eats fishes. It comes ashore for breeding and lays up to 200 eggs. Chelone mydas and Chelone virgata are found in the Bay of Bengal, the upper surface is marbled olive and the lower is pale yellow. In the plastron there is an intergular scute between two gulars.
Trionyx (Fig. 24.2) is commonly known as tortoise or soft river terrapin. It has many species distributed in rivers of Asia, Africa and N. America. The shell has no horny scales but a covering of soft skin. Carapace and plastron are not joined. Trionyx gangeticus is found in the rivers of N. India, its shell is 60 cm long and is olive above and yellow below.
Head is pointed having a black longitudinal stripe, there is no horny beak but lips are fleshy. Carapace has two nuchal bony plates. Each limb bears three claws. Emyda is a soft-shelled terrapin of Indian rivers in which the ribs extend beyond the costal plates of carapace. Kachuga has several species in rivers of India ranging from 15 cm to 55 cm.
Testudo is commonly called land tortoise. It has a wide distribution in tropical and temperate regions of Asia, Africa and Europe. It is terrestrial, diurnal, and herbivorous though at times it also feeds on worms and insects. It hibernates underground in cold weather.
It is known to live up to 100 years or more. The shell is ovoid with well-developed horny scales, when withdrawing its head into the shell, the neck is bent like an S. Feet are for walking on land, the number of phalanges is reduced to two in each digit, digits end in claws. Testudo elegans is found in India and Sri Lanka in dry grassy places, it goes to water in hot weather and hibernates in cold season.
It is about 30 cm long, the males being smaller than females. Carapace is very convex with no nuchal plate, plastron is concave, on the shell are large yellow stars both on central and marginal scales. Tortoises are turtles which have become adapted for terrestrial life, they can live without water for long periods.