A tadpole (also called pollywog or porwigle) 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.
General description 
Tadpoles are young amphibians that usually live in the water, though some tadpoles may be terrestrial. 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 fish.
As a tadpole matures, it most commonly metamorphosizes by gradually growing limbs (usually the legs first, followed by the arms) 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 accommodate the new diet. Most tadpoles are herbivorous, subsisting on algae and plants. Some species are omnivorous, eating detritus and when available, smaller tadpoles.
The name "tadpole" is from Middle English taddepol, made up of the elements tadde, "toad", and pol, "head" (modern English "poll"). Similarly, "polliwog" is from Middle English polwygle, made up of the same pol, "head" and wiglen, "to wiggle".
Fossil record 
Despite their soft-bodied nature and lack of mineralised hard parts, fossil tadpoles (around 10 cm in length) have been recovered from Upper Miocene strata. They are preserved by virtue of biofilms, with more robust structures (the jaw & bones) preserved as a carbon film. In Miocene fossils from Libros, Spain, the brain case is preserved in calcium carbonate, and the nerve cord in calcium phosphate. Other parts of the tadpoles' bodies exist as organic remains and bacterial biofilms, with sedimentary detritus present in the gut. Tadpole remains with telltale external gills are also known from several of the Labyrinthodont groups.
- Zug, G. R.; Vitt, L. J.; Caldwell, J. P. (2001). Herpetology: An Introductory Biology of Amphibians and Reptiles. Academic Press. p. 430. ISBN 978-0-12-782622-6.
- McNamara, M. E.; Orr, P. J.; Kearns, S. L.; Alcalá, L.; Anadón, P.; Peñalver-Mollá, E. (2009). "Exceptionally preserved tadpoles from the Miocene of Libros, Spain: ecomorphological reconstruction and the impact of ontogeny upon taphonomy". Lethaia 43 (3): 290–306. doi:10.1111/j.1502-3931.2009.00192.x.
- McNamara, M. E.; Orr, P. J.; Kearns, S. L.; Alcalá, L.; Anadón, P.; Peñalver-Mollá, E. (2006). "Taphonomy of exceptionally preserved tadpoles from the Miocene Libros fauna, Spain: Ontogeny, ecology and mass mortality". The Palaeontological Association 50th Annual Meeting. The Palaeontological Association.