The sipunculans ("peanut worms") are an exclusively marine group of about 250 known species of round, unsegmented worms with a body consisting of a thick trunk and a slender rectractile proboscis. Sipunculans can be found from the intertidal zone to depths of more than 5,000 meters. Sipunculans range from less than a centimeter to around 50 cm in length, but most are in the range of 3 to 10 cm long. The mouth opens at the front of the proboscis and is surrounded by lobes or tentacles. The gut is U-shaped and highly coiled. The inconspicuous anus opens on the upperside of the body near the front of the trunk. In nearly all species, sexes are separate but not distinguishable externally. Males spawn first and the presence of sperm in the water subsequently stimulates the release of eggs by females. Fertilization is external and development may be direct or indirect (through metamorphosis of a swimming trochophore larva, and in some species involving a second larval stage as well). At least some species are capable of reproducing asexually by dividing into a small posterior fragment and a larger anterior portion, both of which then regrow the missing parts. Most sipunculans are burrowers in substrates that range from mud to coral-rock, although some may seek shelter in other refuges such as snail shells. Under normal conditions, the fluid in the body cavity (coelom) is nearly isotonic to the surrounding seawater (Gosner 1978; Brusca and Brusca 2003 and references therein)
The exact phylogenetic position of the sipunculans has been elusive. Hyman (1959) elevated the group to phylum status, a treatment that stood for many decades, but in recent years molecular phylogenetic analyses have provides strong evidence that the sipunculans may actually be nested within the phylum Annelida (e.g., Struck et al. 2007; Mwinyi et al. 2009; Dordel et al. 2010).