Sipunculans

Sipunculans, commonly known as peanut worms, are a phylum of small, marine, worm-like animals that are unsegmented and lack a true skeleton. They are named for their ability to retract the introvert, the anterior portion of the body bearing a crown of tentacles, into the trunk, producing a peanut-like shape when contracted.

The body consists of an introvert and a trunk. The introvert can be everted for feeding and defense and bears numerous tentacles that capture organic particles from water or sediment. The trunk houses a coelom with a hydrostatic skeleton and the digestive system. The gut is U-shaped, with the mouth at the end of the introvert and the anus near the posterior trunk. Sipunculans lack segmentation and do not have specialized circulatory or respiratory organs; gas exchange occurs across the body wall.

Sipunculans inhabit marine environments, living in burrows in soft sediments, under rocks, or among coral rubble and seagrass. They are primarily deposit feeders, using their tentacular crown to collect detritus, bacteria, and microalgae. Most species are dioecious, releasing eggs and sperm into the water for external fertilization; development is often indirect, with a free-swimming larval stage called pelagosphera before metamorphosis into the adult form. The classification of Sipuncula remains debated, with molecular studies suggesting a close relationship to annelids, though many authorities continue to treat Sipuncula as a separate phylum.

Sipunculans contribute to benthic ecosystem processes such as sediment turnover and nutrient cycling and are common in many coastal and continental-shelf habitats.