The class name refers to the creeping or crawling mode of locomotion (Latin repere or reptum: to creep or crawl). Reptiles are mostly terrestrial animals, and their body is covered by dry and cornified skin with epidermal scales or scutes. They do not have external ear openings; a tympanum represents the ear. Limbs, when present, are two pairs, and limbs are absent in snakes. The heart is usually three-chambered, but it is four-chambered in crocodiles. Reptiles are poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes are separate. Fertilisation is internal. They are oviparous and development is direct. Examples include Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), Calotes (garden lizard), Crocodilus (crocodile), Alligator (alligator) and Hemidactylus (wall lizard), along with poisonous snakes such as Naja (cobra), Bangarus (krait) and Vipera (viper). Reptiles belong to Tetrapoda. The points that are most frequently tested include the dry cornified skin, shedding of scales as skin cast, three-chambered heart with the exception of the four-chambered heart in crocodiles, internal fertilisation, direct development and the poikilothermous condition, which together separate reptiles from amphibians, whose skin is moist and whose development is indirect.