The aquatic Oligochaeta as well as the earthworms can be derived from ancestors closely resembling living Haplotaxidae. Four of the five required intermediate forms are represented by living haplotaxid species. The polychaetes are also thought to be derived from simple earthworm-like forms. These concepts are in accord with recent locomotor theory separating the evolution of the coelom and segmentation as a two-step process related to sustained burrowing activity, as opposed to the earlier complex of theories involving the simultaneous evolution of coelom and segmentation. Unsegmented coelomates are seen as representatives of an intermediate condition between acoelomates and segmented coelomates instead of problem phyla derived by degeneration of segmented ancestors.