Successive stages of the evolutionary transformation of the atrochal larva into a trochophore. A, A completely ciliated larva. B, Larva with an apical tuft and an equatorial band of cilia. C, Larva with mouth and anus. D, Larva with the ciliation restricted to a prototroch and a perioral ciliation. E, Larva with prototroch, adoral ciliary zone and metatroch. Modified from .