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.
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