Aquatic larval stage of frogs and toads. Tadpoles have a short, oval body, broad tail, small mouth, and no external gills. Most are herbivorous, but those of some species are predatory or even cannibalistic. Tadpole metamorphosis follows a pattern of gradual development of forelimbs and hind limbs, resorption of the tail, shortening of the intestine, disappearance of the gills, and development of the lungs. On completion of metamorphosis, the young frog or toad emerges onto land.
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A tadpole or polliwog (also pollywog, polliwig,, polewig, or polwig) is the wholly aquatic larval stage in the life cycle of an amphibian.
During the tadpole stage of the amphibian life cycle, most respire by means of autonomous external or internal gills. They do not usually have arms or legs until the transition to adulthood, and typically have dorsal or fin-like appendages and a tail with which they swim by lateral undulation, similar to most fishes.
As tadpoles mature, they most commonly metamorphosize by gradually growing limbs and then (most commonly in the case of frogs) outwardly absorbing its tail by apoptosis. Lungs develop around the time of leg development and tadpoles late in development will often be found near the surface of the water where they breathe air. During the final stages of external metamorphosis, the tadpole's mouth changes from a small enclosed mouth at the front of the head to a large mouth the same width as the head. The intestines shorten to make way for the new diet. Most tadpoles are herbivorous, subsisting on algae and plants. Some species are omnivorous, eating detritus and, whenever available, other tadpoles.
An exception to the rule distinct differences between the tadpole (juvenile) and adult (frog, toad, salamander etc) stages is the axolotl. Axolotls exhibit a property called neoteny, meaning that they reach sexual maturity without undergoing metamorphosis.