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Phylum Sipuncula (sigh-pun + kyu-la) (L. sipunculus, little siphon) consists of about 250 species of benthic marine worms, at depths ranging from the intertidal to over 5000 m. They live sedentary lives in burrows in mud or sand, occupy borrowed snail shells, or live in coral crevices or among vegetation. Some species construct their own rock burrows by chemical and perhaps mechanical means. More than half of the species are restricted to tropical zones. Some are tiny, slender worms, but the majority range from 3 to 10 cm in length. Some are commonly known as “peanut worms” because, when disturbed, they can contract to a peanut shape.
Sipunculans have no segmentation or setae. They are most easily recognized by a slender retractile introvert, or proboscis, which is continually and rapidly being run in and out of the anterior end. Walls of the trunk are muscular. When the introvert is everted, the mouth can be seen at its tip surrounded by a crown of ciliated tentacles. Little is known about the details of sipunculan feeding. Some species appear to be deposit feeders or detritivores, whereas others appear to be suspension feeders. Some nutrition may also come from dissolved organic compounds directly from the water column.
Undisturbed sipunculans usually extend the anterior end from their burrow or hiding place and stretch out their tentacles to explore and to feed. Organic matter collected in mucus on the tentacles is moved to the mouth by ciliary action. The introvert is extended by hydrostatic pressure produced by contraction of body-wall muscles against the coelomic fluid. The lumen of the hollow tentacles is not connected to the coelom but rather to one or two blind, tubular compensation sacs that lie along their esophagus. These sacs receive fluid from the tentacles when the introvert is retracted. Retraction is effected by special retractor muscles. The surface of the introvert is often rough because of surface spines, hooks, or papillae.
There is a large, fluid-fi lled coelom traversed by muscle and connective-tissue fibers. Their digestive tract is a long tube that doubles back on itself to form a U-shape and ends in an anus near the base of the introvert. A pair of large nephridia opens to the outside to expel waste-filled coelomic amebocytes; the nephridia also serve as gonoducts. Circulatory and respiratory systems are lacking, but coelomic fluid contains red corpuscles that have a respiratory pigment, hemerythrin, used in transportation of oxygen. Gas exchange appears to occur largely across the tentacles and introvert. Their nervous system has a bilobed cerebral ganglion just behind the tentacles and a ventral nerve cord extending the length of the body.
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