Chordate Classes II — Amphibia, Reptilia, Aves and Mammalia

Class Amphibia (Greek amphi, both + bios, life) comprises animals capable of living both in water and on land, a dual existence reflected in the class name. The skin is smooth, moist and glandular (rich in mucus glands that keep it from drying out), and, notably, is entirely without scales. Respiration changes over the life cycle and may involve gills (in the aquatic larval, or tadpole, stage), lungs (in the terrestrial adult) and the moist skin itself (cutaneous respiration, important throughout life in many species). The heart is three-chambered (two atria, one ventricle), and, like every non-avian, non-mammalian vertebrate group, amphibians are poikilothermous (cold-blooded), their body temperature varying with the surrounding environment rather than being internally regulated. The sexes are separate, fertilisation is usually external, and development is indirect, passing through an aquatic larval stage that undergoes a dramatic metamorphosis into the terrestrial (or semi-terrestrial) adult form. Familiar examples include Bufo (the toad), Rana (the frog), Hyla (the tree frog) and the limbless, burrowing Ichthyophis.

Class Reptilia (Latin repere, to creep) comprises crawling or creeping, chiefly terrestrial animals whose dry, cornified (keratinised) skin, covered by epidermal scales or bony scutes, is a major adaptation limiting water loss on land, in sharp contrast to the moist amphibian skin. External ear openings are absent, and, like amphibians, reptiles are poikilothermous. The heart is typically three-chambered, though crocodilians are a notable exception with a fully four-chambered heart, foreshadowing the birds and mammals. Fertilisation is internal, and the fertilised egg is enclosed in a protective calcareous shell (the amniotic egg) that is laid on land, so that, unlike amphibians, reptiles have entirely broken away from a required aquatic larval stage. Familiar examples include Chelone and Testudo (turtles and tortoises), Chameleon, Calotes (the garden lizard), Crocodylus, Alligator, Hemidactylus (the common wall lizard) and Naja (the cobra).

Class Aves (birds) is defined above all by feathers, unique to this class among living vertebrates, which cover the body and provide insulation as well as the surface needed for flight. The forelimbs are modified into wings (though not every bird can fly — ostriches and penguins are notable flightless examples), and the hindlimbs are variously adapted for walking, perching, wading or swimming, ending in a beak rather than teeth. Birds are homoiothermous (warm-blooded), maintaining a constant, typically high, body temperature regardless of the surroundings, an ability shared, among the classes in this chapter, only with the mammals. The heart is completely four-chambered, ensuring the fully separate circulation of oxygenated and deoxygenated blood needed to fuel a high, constant metabolic rate, and respiration is assisted by a system of air sacs that supplement the lungs. Fertilisation is internal, all birds are oviparous, and parental care of the eggs and young is a nearly universal feature of the class. Corvus (the crow), Columba (the pigeon), Struthio (the ostrich), Aptenodytes (the penguin) and Pavo (the peacock) are familiar examples. …