The phylum Sipuncula comprises worm-like, marine animals occurring both intertidally and sublittorally. They are found in burrows in sandy and muddy sediments, in the empty tubes of polychaetes, the empty shells of molluscs, and in rock crevices. Sipunculans have an unsegmented body in which the anterior part, or introvert, is narrow and can be withdrawn, and the posterior part, the trunk, which is wider and more or less cylindrical. In relaxed specimens the mouth is seen at the anterior end of the introvert surrounded completely, or in the dorsal aspect, by lobes or tentacles. The anus is at the anterior end of the trunk in a mid-dorsal position. Sipunculans are mainly deposit feeders, feeding on mud, sand and detritus but filter feeding, using a crown of modified tentacles, has been described in one group. Almost all species have separate sexes with external fertilization, but see Nephasoma minutum (p. 377). There is a free-swimming larva and the length of pelagic life varies from a few days to a month or so.

Identification to species can be difficult and often requires examination of internal anatomy. The species described here can be recognized on external morphology and habitat.