Fauna associated with the sipunculid Phascolion strombi (Montagu), especially the parasitic gastropod Menestho diaphana (Jeffreys)

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Abstract
The fauna associated with Phascolion strombi from near Kristineberg (Swedish west coast) comprises two groups. 1) Fauna on the exposed surface of the shelter. Nineteen species were found and interaction with the sipunculid was in most cases found to be neutral. 2) Fauna inside the shelter. Several species of polychaetes, sipunculids and a halacarid were found in the clay plugs of the shelters, but the occurrence of these animals seems to be more or less accidental. The pyramidellid Menestho diaphana was common. It is an obligatory parasite of Phascolion. Its feeding mechanism is described with special emphasis on the role of the stylet. Egg masses were found in summer and the larvae hatched from these are described and figured. The adult snails seem usually to die after spawning. Two species of entoprocts were found on the cuticle of Phascolion. Their feeding is described. A hydroid was found on the anterior papillated region of some Phascolion. Its occurrence seems to be accidental. The five types of interaction between Phascolion and its associated fauna are listed in a table.