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Characteristics, distribution and ecology
Taxonomische classification
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(Keferstein, 1862)

It is a small species, not exceeding 15 mm in length, with a cylindrical trunk. The oral disk carries a rudimentary tentacular crown consisting of a dorsal pair of short tentacles plus 2-6 rounded lobes or prominences which tend to be rather variable in form according to the degree of eversion. Simple, spine-like hooks may be present on the anterior introvert; they are usually fairly numerous on juveniles and regenerated specimens but are few, or entirely absent, on adults, presumably through wear. When present, hooks are arranged irregularly. The skin of the introvert and trunk is smooth but minute papillae may be discernible, especially over the posterior trunk region. The nephridiopores are lateral and just posterior of the anus on the anterior trunk. The longitudinal muscle layer of the body wall is continuous, not collected into bands.
Internally, two retractor muscles are present, inserted in the middle third of the trunk; often the two retractors are fused for most of their length (N. minutum-internal). The intestine is tightly coiled in a double spiral supported by a spindle muscle attached anteriorly but not posteriorly. The rectal caecum is usually conspicuous. Typically, one gut-fixing muscle fastens the oesophagus to the body wall on the left side.
This species is unusual, perhaps unique, amongst the Sipuncula in being hermaphroditic. Both sperm cells and oocytes are usually detectable in the coelom throughout the year. At Plymouth, it spawns from November to January. Sterility is caused by a parasitic copepod, Akessonia occulta occurring singly in the coelom.
As a result of morphological variability, especially in the presence or absence of hooks and papillae, the taxonomy of N. minutum is very confused.

Inhabits mud and sand, from the mid-tide level to about 50 m depth. Common in rock crevices, amongst Sabellaria tubes and similar niches.

A widespread species in European waters from Shetland and Sweden to Brittany.

Nephasoma minutum