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During the tadpole stage of the life cycle the tadpole breathes by means of autonomous external or internal gills. It does not have arms or legs or reproductive organs until later stages in its life, and has a finned tail with which it swims by lateral undulation, similar to most fish. As a tadpole matures, it metamorphoses by gradually growing limbs and then (in the case of frogs) absorbing its tail by apoptosis. Lungs develop around the time of leg development, and a tadpole late in development will often swim to the surface of the water to breathe air. During the final stages of external metamorphosis, the tadpole's mouth must change from a small round mouth at the front of the head, to a large mouth the same width as the head. The intestine will shorten to accommodate the frog's carnivorous diet.