Sipunculans are worm-like animals ranging from 2 to 720 millimetres (0.079 to 28 in) in length, with most species being under 10 centimetres (3.9 in). The body consists of a narrow front section, called the introvert and a larger trunk section. The introvert is retractable into the trunk. 
Sipunculans have a coelom (body cavity), and a body wall somewhat similar to that of annelids. The body wall is strong and muscular. When threatened, Sipunculids can retract their body into a shape resembling a peanut kernel – a practice that has given rise to the name "peanut worm".
The most recognizable part of Sipunculan worms is their mouth, which is surrounded by a mass of 18–24 ciliated tentacles, all of which may be inverted into the body. The tentacles are used to gather organic detritus from the water or substrate, and are probably also function as gills. The mouth and tentacles lie at the tip of the introvert, which is otherwise decorated with numerous spines or nodules. In contrast, the trunk tends to be relatively smooth.