Tadpole

A tadpole (also called a pollywog) is the larval stage in the life cycle of an amphibian, particularly that of a frog or toad. They are usually wholly aquatic, though some species have tadpoles that are terrestrial. When first hatched from the egg they have a more or less globular body, a laterally compressed tail and internal or external gills. As they grow they undergo metamorphosis, during which process they grow limbs, develop lungs and reabsorb the tail. Most tadpoles are herbivorous and during metamorphosis the mouth and internal organs are rearranged to prepare for an adult carnivorous lifestyle.

Having no hard parts, it might be expected that fossil tadpoles would not exist. However, traces of biofilms have been preserved and fossil tadpoles have been found dating back to the Miocene. Tadpoles are eaten in some parts of the world and are mentioned in folk tales and used as a symbol in ancient Egyptian numerals.

The name "tadpole" is from Middle English taddepol, made up of the elements tadde, "toad", and pol, "head" (modern English ""). Similarly, "pollywog" / "polliwog" is from Middle English polwygle, made up of the same pol, "head", and wiglen, "to wiggle".

Tadpoles are young amphibians that usually live in the water, though a few tadpoles are semi-terrestrial (Indirana beddomii and Thoropa miliaris) and terrestrial (Indirana semipalmata and Adenomera andreae). 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 a large, flattened tail with which they swim by lateral undulation, similar to most fish.

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