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A phylum of sedentary marine vermiform coelomates that are unsegmented, but possibly distantly related to the annelids; they are commonly called peanut worms. Two classes are defined: Sipunculidea and Phascolosomatidea. In all there are 17 genera and approximately 150 species living in a wide variety of oceanic habitats within the sediment or inside any protective shelter such as a discarded mollusk shell, foraminiferan test, or crevice in rock or coral.

Adult sipunculans range in trunk length from 2 to over 500 mm (0.08 to over 20 in.). The shape of the body ranges from almost spherical to a slender cylinder. Sipunculans have a variety of epidermal structures (papillae, hooks, or shields). Many species lack color, but shades of yellow or brown may be present. Internal anatomy is relatively simple. The digestive tract has a straight esophagus and a double-coiled intestine extending toward the posterior end of the body and back terminating in a rectum, sometimes bearing small cecum. A ventral nerve cord with lateral nerves and circumenteric connectives to the pair of cerebral ganglia are present. Two or four pigmented eyespots may be present on the cerebral ganglia, and a chemoreceptor (nuchal organ) is usually present.

Knowledge of the reproductive biology of sipunculans is scanty, and good information on breeding cycles is unavailable for most genera. Most sipunculans are dioecious and lack any sexual dimorphism. These worms play a part in the recycling of detritus and probably consume smaller invertebrates in the process. They are in turn preyed on by fishes and probably other predators (including humans).

References in periodicals archive ?
1968): On the distribution of new sipunculid fauna of the littoral of Cuba and the Gulf of Mexico.
Similar granules, containing enzymes implicated in bacterial killing or in intracellular digestion, have also been described in the coelomic granulocytes of a different sipunculid worm (Dybas, 1981a, b) and in tunicate hemocytes (Sawada et al.
Because the incubation medium we used contained 2% of sipunculid coelomic fluid, the possible contribution of third-type cells and other circulating cellular constituents to tail formation must be investigated further.
Goatfishes are unique in having a pair of sensory barbels on the chin that are used to probe into sedimentary substrata to detect fossorial prey, mainly a variety of crustaceans, polychaetes, sipunculids, small bivalves, and occasional small fishes or small octopuses.
some polychaetes, sipunculids, vestimentiferans, echiurans, entoprocts, and molluscs).
Echiuran worms and sipunculids were identified by the presence of chetae in the digestive tract (Fisher 1946, Brusca 1980, Brusca & Brusca 2005).
Grazing gastropods and deposit-feeding sipunculids were used to establish that bacterial chemoautotrophic production at GC185 ranged in [[delta].
Examples in which such cell groups are lacking are found within the larvae of sipunculids (Wanninger et al.
1972) and Strathmann (1978)--bivalves, annelids, echiurids, sipunculids, and entoprocts.
The highly stereotypic cleavage pattern referred to as "spiral cleavage" occurs in most of the extant invertebrate phyla, including the molluscs, annelids, vestimentiferans, pogonophorans, echiurans, sipunculids, nemerteans, gnathostomulids, mesozoans, and polyclad turbellarians.
The Spiralia comprises several invertebrate phyla including the molluscs, annelids, nemerteans, sipunculids, echiurans, and some turbellarian platyheiminths.