Long axis of base up to 100 mm, span of tentacles to 50 mm.
Base and lower part of the column expanded laterally, forming two lobes enveloping a hermit crab and its gastropod shell so that the disc is beneath the crab with the two lobes meeting on its dorsal side. Base capable of secreting a chitinous membrane which effectively increases the capacity of the shell so that as the crab grows it has no need to change its abode.
Upper part of column short and cylindrical. Cinclides are present on low mounds on the lower part of the column, from which long acontia are profusely and freely emitted at the slightest provocation.
Colouration: Column, disc and tentacles white, the lower part of the column more or less suffused with chestnut. Column usually spotted with lilac-pink or crimson and a line of the same running around its upper margin; acontia usually pink, rarely white.
Usually found on substrata of sand or gravel, especially where stones or broken shells are present, or near rock outcrops, etc. Typically a sublittoral species occurring down to 200 m or more but occasionally found on the lower shore where it may he washed up after storms. Young specimens, of normal anemone-shape but possessing the characteristic pink spots of the adult, may occur on rocks or shells between tidemarks. It lives almost exclusively on the hermit crab Pagurus prideauxi (Leach) in British waters, although other species may be involved elsewhere.
Common around all coasts of Britain and western Europe in general, from Norway to the Mediterranean.