A series of different behavioural reactions contribute to the sequence of acts by which the commensal ragworm N. fucata gains entry into sheik occupied by its host, the hermit crab. Mechanical and chemical stimuli from the host trigger a bout of general activity during which a number of different acts may be performed but at the same time they facilitate searching so that this act is usually performed soon after the detection of the crab. Tactile contact with the crab's shell elicits two short‐latency responses: worms vacate their tubes and climb on to the shell on first touching it and then crawl into the shell on reaching the lip of the aperture. An orthokinesis is also evident in the worm's behaviour. All of these reactions are relatively unspecific, and can be elicited by stimuli from a non‐host animal, the shore crab C. maenas.