Biology/Natural History: Length (with introvert extended) to 12 cm. This is the most common sipunculan in California. In the San Juan Islands it breeds from June to September. When gravid, gametes may make up to 37% of animal's mass. The eggs are yellow or orange, flattened and beanlike and surrounded by a thick envelope. The planktotrophic larva spends a long time in the plankton. This species has cerebral eyes which are similar to those of flatworms, annelids, and mollusks. This species uses its 18-24 short, simple tentacles around the mouth to collect detritus from the surface of the sediment, then inverts the introvert and swallows the detritus. Black Oystercatchers are said to eat this species on Vancouver Island.

Members of Genus Phascolosoma have their longitudinal muscles in 4 bands rather than in a continuous column around the body as seen in most other Sipunculans. These bands can sometimes be seen externally. Sipunculan coelomic fluid contains unusual motile, multicellular structures called urns. The urns gather metabolic and particulate wastes, produce mucus in response to infection, and help the blood to clot after an injury. The urns can easily be seen in the coelomic fluid under a microscope. Sipunculans such as Phascolosoma have separate sexes. Their eggs or sperm are produced from cells in the peritoneum surrounding the coelom rather than in distinct gonads. They mature in the coelomic space and then are shed out the nephridia (kidneys). Males spawn first, which seems to trigger females to spawn. The fertilized egg develops into a trochophore larva (similar to polychaetes and mollusks) which feeds off internal food, then (in Phascolosoma) elongates and becomes a pelagosphera larva which feeds. The pelagosphera eventually elongates more, settles, and grows up into a juvenile then adult.