GENERAL EXTERNAL ANATOMY
Anatomical modifications have evolved according to the habitat and whether it is terrestrial, marine, or freshwater.
Terrestrial (tortoises)
These are high-domed, stump-legged, small-headed vegetarians (Fig.
3.3) that are found in hot and arid climates (Evans 1986; Hoffstetter & Gasc 1970; Pough 1998a). They have developed thick skin and large scales to prevent desiccation. Largest of the land tortoises include the Galapagos tortoise (Geochelone nigra) (Fig. 3.2) and the Aldabra (Geochelone gigantea), but they mainly range from 10 to 120 cm in size. Many smaller species, like the North American Gopher (Gopherus polyphemus) tortoise, have adapted for burrowing with a flatter dome and scoop-shaped forelegs, while the African Pancake tortoise (Malacochersus tornieri) has a shell so flat and flexible it can slide between rock crevices to escape predators (Pough et al. 2002).Aquatic
Chelonia are well modified for aquatic existence as they can easily breathhold, and utilize anaerobic metabolism and non-pulmonary respiration through skin, pharynx or cloaca. They also have the ability to shunt blood away from the lungs while diving and tolerate severe acid-base imbalances.
Hibernation
Chelonians from subtropical or temperate areas hibernate in winter months under the influence of decreasing temperatures. Terrestrial species bury themselves below the frostline underground while some aquatic species bury themselves
CLINICAL NOTE
Semi-aquatic turtles, like the Red-eared slider, bask on dry land to aid digestion of food. However, basking in the sun may also play a role in ecdysis and prevents fungal infections by drying out the shell (Pough et al. 2002).
Marine
Marine species are the largest of all and have evolved a flatter, softer shell for better streamlining (Fig. 3.4). The metacarpal/tarsal bones and phalanges are elongated for swimming and are similar to flippers.
The fore flippers are long and oar-like and used for propulsion while the hind flippers are used for steering and digging the nest. The Leatherback turtle (Dermochelys coriacea) is the largest, the Hawksbill turtle (Eretmochelys imbricata) the smallest (Evans 1986; Pough et al. 2002).Freshwater
These are aquatic or semi-aquatic, have webbed feet and a smooth shell. Softshell turtles (Trionychidae) from North America and Asia are strong swimmers and mainly carnivorous. They are the most aquatic of all freshwater species
Table 3.1 Taxonomy and classification of Chelonia (Rough et al. I998a, 2002)
| Suborder | Family | Location | Examples |
| Rleurodira | Chelidae | Australasia, S. America | Matamata |
| Relomedusidae | Africa, S. America | River turtle | |
| Cryptodira | Carettochelyidae | Australasia | Rig-snouted river turtle |
| Cheloniidae | Marine | Green turtle, Hawksbill turtle | |
| Chelydridae | North America, Central America | Snapping turtle | |
| Dermatemydidae | Central America | River turtle | |
| Dermochelyidae | Marine | Leatherback turtle | |
| Emydidae | N & S. America, Europe, Asia, Africa | Box, Common slider, European pond terrapin | |
| Kinosternidae | North America | Mud and musk turtles | |
| Rlatysternidae | Southeast Asia | Big-headed turtle | |
| Staurotypidae | Central America | Mexican musk turtle | |
| Testudinidae | Cosmopolitan, except Australia | Herman’s tortoise, Leopard tortoise | |
| Trionychidae | N. America, Africa, Asia | Softshell turtles |
GENERAL INTEREST