The first body segment is twice as long as the rest and bears the pharynx and four pairs of tentacular cirri.

[2] Over much of its range, the normal host is the common European species Pagurus bernhardus which is found on the mid and lower shore and at depths down to about 140 m (460 ft).

[3] Neanthes fucata lives inside an empty gastropod mollusc shell inhabited by a hermit crab.

However the worms are gonochoristic and there must be some mechanism by which a male and female come into close proximity during the period in which they are reproductively active.

Some possibilities for this to happen include the hosts being close together, temporary aggregations of individual worms during the breeding season, or by the release of gametes into the sea water.