A remarkable association, most correctly designated as commensalism, is described between
Terebellides stroemi and a small polychaete of the family Sphaerodoridae. A description is given of the commensal species which is new to science, together with some scattered observations on the commensal-host relation; the habitat of the commensal seems to be the tentacle crown of the host and it most probable feeds on the food particles strolled along the tentacles. The taxonomy of the Sphaerodoridae is revised with the conclusion that there exist two distinct genera, one of which for nomenclatural reasons should have its name changed, and it is proposed to employ the name
Sphaerodoridium for this genus which among others includes the commensal species.
