amphibian

The life cycle of a frog. Frogs lay their eggs (spawn) in fresh, still water. The tadpoles hatch within about two weeks and then over the course of the next three months they grow larger and slowly develop legs and internal gills. By the time they lose their tails they resemble tiny adult frogs and can leave the water.

The frog is an unusual vertebrate in that it has no ribs.

Member of the vertebrate class Amphibia, which generally spend their larval (tadpole) stage in fresh water, transferring to land at maturity (after metamorphosis) and generally returning to water to breed. Like fish and reptiles, they continue to grow throughout life, and cannot maintain a temperature greatly differing from that of their environment. The class contains about 4,500 known species, 4,000 of which are frogs and toads, 390 salamanders, and 160 caecilians (wormlike in appearance).

According to analysis of statistics from over 900 amphibian populations taken 1950–97, by Canadian and Swiss researchers, and published in 2000, amphibian populations have been declining at a rate of about 4% per year. US biologists concluded that the marked worldwide decline in amphibian populations is unlikely to have a single cause. Possible causes have been cited as increased ultraviolet radiation exposure, pollution, global climate change, and infection.