THESE are cold-blooded, air-breathing vertebrates which may be regarded as the earliest types of vertebrates to have become well adapted for life on land. Unlike the amphibians, which resort to water to lay their eggs and which pass through a fish-like larval stage before developing in the adult form, reptiles lay large yolky eggs protected by calcareous shells. Inside these the embryo animals develop into miniature adults,on the food provided in the eggs. A free-living, self-supporting larval stage does not therefore occur. Internal fertilization takes place by the male introducing sperm into the oviducts of the female by an act called coition, so that the eggs are fertilized before receiving the calcareous shell prior to their being laid.
Reptiles are characterized by their complete covering of dry epidermal scales and the presence in their jaws of teeth, all of the same simple type. Only in crocodiles and alligators are the teeth set in sockets as in mammals. Most reptiles have two pairs of pentadactyl limbs which do not fully support the body from the ground. They therefore sprawl and wriggle along the ground, using their tails as well as their limbs as organs of propulsion. The digits are clawed.
Reptiles include such animals as lizards, snakes, crocodiles and tortoises.