Amphibians, reptiles, birds and mammals

Amphibia (amphi = dual, bios = life) live both on land and in water. Their skin is moist, with no scales. They have eyelids and a tympanum (eardrum on the surface). Gut, urine and reproductive tracts open into one chamber, the cloaca. They breathe through gills, lungs and the skin. The heart has three chambers. They are cold-blooded, and fertilisation is usually outside the body. Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra, Ichthyophis (limbless).

Reptilia (from repere, to creep) are mostly land animals with dry, horny skin covered in scales or scutes. They have no external ear openings. The heart usually has three chambers, but the crocodile has four. They are cold-blooded (poikilotherms). Snakes and lizards shed their scales as a skin cast. Fertilisation is internal, and they lay eggs. Chelone (turtle), Testudo (tortoise), Calotes (garden lizard), Crocodilus, Naja (cobra), Vipera (viper).

Aves (birds) have feathers and a beak, and their forelimbs are wings. Their bones are hollow and air-filled (pneumatic), keeping them light for flight. The only skin gland is an oil gland at the tail. The gut has a crop (to store food) and a gizzard (to grind it). Air sacs help the lungs. The heart has four chambers, and they are warm-blooded (homoiothermous). Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), Neophron (vulture).

Mammalia are named after their most special feature: mammary glands, which make milk for the young. They have hair, external ears (pinnae), and different kinds of teeth. The heart has four chambers and they are warm-blooded. Most give birth to live young (viviparous). Exception: Ornithorhynchus (platypus) lays eggs. Others: Macropus (kangaroo), Pteropus (flying fox), Delphinus (dolphin), Balaenoptera (blue whale).

Body heat sums it up: fishes, amphibians and reptiles are cold-blooded (their body temperature changes with their surroundings); birds and mammals are warm-blooded (they keep it steady).