As well as covering the shell, the mantle lobes (1) extend beyond the shell to create a peripheral cavity that covers the head and foot (2). When not in feeding position, the head (3) is flat and rectangular with long, slender, smooth cephalic tentacles on the anterior corners and no visible snout projecting between the tentacles. On the ventral face of the head there is an opening to a sac containing an inverted white proboscis. When ready to feed, the proboscis (4) is engorged and everted from the opening. After further elongation, its mouth is applied to colonial or unitary ascidians for the radula within the proboscis to cut out pieces of test.
Behind the right tentacle, males have a relatively enormous, pale penis (5) with the genital duct protruding like a spur (6) near the distal end.

Male. Length 25mm, Llŷn Peninsula, Wales. March 2016.