The animals of the class Amphibia are the most primitive of the land-dwelling vertebrates. There are three orders in the class: the Caudata, or tailed amphibians, which includes the sirens (top, left), salamanders (top, right), and newts; the Anura, or tailless amphibians, which includes the frogs and toads (bottom, left); and the Gymnophiona, which includes the highly modified, wormlike amphibians called caecilians (bottom, right). Although all amphibians spend much of their adult lives in or near water, some such as the toads, salamanders, and newts may spend considerable periods on land.
Nuridsany & Perennou/Photo Researchers, Inc. Tom McHugh/Photo Researchers, Inc. Dorling Kindersley Michael
The legless tadpoles that hatch from a floating mass of frog eggs are the animal’s fishlike larval stage. Part of a true metamorphosis, they have gills and a tail, both of which disappear as the tadpole feeds and grows. When limbs and air-breathing lungs develop, the young frog, now a miniature replica of its parents, emerges from water to land.
