Up to 17mm long. Ample mantle covers whole body. Usually yellow (EML) or yellow-orange, but can be pure white , especially in northern Scotland. Usually no surface pigment. Club-shape, or ellipsoid, tubercles  with obtusely pointed apex.
Apex truncated. Up to 18 front-parted oblique lamellae  on 14mm long animal, occasionally 19 on larger specimens. About 12 lamellae on 8mm specimens. Colour as mantle, but often a darker shade on yellow specimens, and yellowish white on white specimens.
Up to 12 translucent gills (EML) coloured as mantle, arranged in a horseshoe around the anus.
Large oral veil with large oral orifice.
Sole translucent, coloured as mantle, showing purplish liver. Anterior of foot slightly truncated with grooved edge, not indented by mouth. Usually completely concealed dorsally by mantle, but posterior tip may sometimes protrude.
On lower shore and sublittorally to 60m. Feeds on wide variety of polyzoans, including Electra pilosa [EML] (often on Fucus serratus), Membranipora membranacea [EML] (often on Laminaria), Flustrellidra hispida [MeL] and Alcyonidium polyoum. Simultaneous hermaphrodite. Cream coloured spiral ribbon of spawn laid February to May. Adults die after spawning so inconspuous until next generation grows. Veliger larvae before transforming into adult form.