The sipunculan body consists of a plump, unsegmented trunk and a more slender introvert, which is extended from, and withdrawn into, the anterior end of the trunk (External characters). The anterior tip of the introvert, or the oral disk, usually has a lobed nuchal organ on its dorsal side and carries mucus-covered tentacles (Oral disk). The shape, number and arrangement of the tentacles vary in different genera and species. Generally, they are flattened or digitiform in shape and increase in number with growth. These tentacles trap particles from the surrounding water or are pressed into the substrate, trapping mud and detritus. The entire introvert may then be withdrawn into the trunk and the captured particles ingested, or the particles may be moved to the centrally placed mouth by ciliary tracts on the tentacles.
Behind the oral disk, a zone of chitinous, posteriorly directed hooks may be present, arranged either in well-marked rings or scattered in an irregular pattern. The hooks may be simple and spine-like, or flat and curved. The anus is situated on the mid-dorsal line towards the anterior end of the trunk (except for species of the genus Onchnesoma , where it has moved to the introvert). At the anterior end of the trunk, one or two nephridopores are present, usually ventrolateral in position. The skin contains groups of glandular cells, forming the epidermal organs. These are often raised to form papillae, which vary in size and shape. In some genera, e.g., Aspidosiphon , the skin is heavily chitinized at one or both ends of the trunk, forming shields. The body wall is composed of two main muscle layers: an outer circular and an inner longitudinal one. These layers may form continuous sheets, or collected into bands.
Internally, the main muscles are the retractors of the introvert (Internal structures). The number and arrangement of the retractor muscles is of generic importance: there may be four retractors arranged in dorsal and ventral pairs; two, one dorsal and one ventral or two in a ventral pair; or only one ventral muscle. The gut consists of three, ill-defined sections: an oesophagus, a long intestine, which is often coiled in a double spiral, and a rectum, generally bearing a rectal caecum. The double spiral of the intestine consists of a descending and an ascending loop and is supported by the spindle muscle. The gut is anchored by fixing muscles and the rectum by a paired wing muscle. The hollow tentacles operate by means of a compensatory system in the form of a contractile vessel or compensatory sacs. These sacs are attached to the surface of the oesophagus; they may be simple in some species, but can form ramifying branches throughout the trunk coelom and forming a type of circulatory system in other species. Contraction of the sac musculature drives fluid into the tentacles, bringing about their extension. Muscular retraction of the tentacles drives fluid back into the sacs. The nephridia are sac-like structures in the anterior trunk region, often attached to the trunk wall by fine strands. Each nephridium opens to the coelom by an anterior nephrosome. The nephridial system is supplemented by a system of urns. These urns are clusters of cells that arise and detach from the coelom wall. Floating freely in the coelomic fluid, the urns collect solid wastes and eventually deposit these wastes in the body wall or exit the animal through the nephridial system. In some genera, only one nephridium (usually the right one) is developed. The nerve cord is mid-ventral in position, running anteriorly to the brain, a bilobed mass situated dorsally behind the oral disk. The gonads appear as fine strands at the base of the ventral retractors; gametes are released to the coelom at an early stage of development and prior to the spawning season, often completely fill the coelom. The free-swimming trochophore larvae are released through the nephridiopores. In some sipunculans, the trochophore larva develops further to form a pelagosphera larva, which at one time was thought to be a free-swimming adult sipunculan and was placed in the now-defunct genus Pelagosphaera . These larvae are typically large (several millimetres long) and long-lived and may disperse over great distances.
Source: Gibbs, 1977.