Varies greatly in form and colour.
Body (excluding appendages)
Frequently 25mm – 60mm long , but up to 90mm, or, exceptionally, 120mm. Flesh mostly opaque; white, brown, yellowish, ginger , rosaceous  or pinkish , powdered and freckled with opaque white and gray, brown or lilac. White anal papilla usually concealed among cerata .
Numerous, arranged in about 20 close rows of 12 – 24 cerata each, on each side of the body, concealing sides, but front half, at least, of dorsum usually exposed . Ceras not flattened, becoming longer and attenuated when active . Anterior cerata small, positioned well in front of rhinophores . Dense pigment, variously coloured, conceals internal yellow digestive gland, except for small translucent area at base of posterior face (only exposed when ceras swept forwards ). Tips whitish.
Smooth, short, straight, conical with trucated apex, and contractile. Opaque, similar colour to body, or darker shade, darkening more when contracted. Apex pale; yellowish or whitish.
Anterior a broad smooth curve between oral tentacles , unless drawn back and puckered . Distance between oral tentacles at base about 3 times thickness of tentacle base. Oral tentacles usually a little longer than rhinophores. Oral tentacles usually distally powdered white and proximally flecked grey . Internal eyes at base of rhinophores not  visible through opaque pigment.
Rather broad, especially at front. Anterior gently convex with deep marginal groove and small distinct propodial tentacles. Sole translucent white, revealing rose tinted ovotestis.
- Rarely a pale V mark in front of rhinophores, more usual A. filomenae (see https://flic.kr/s/aHsme6Cmz5 .
- Length can be greater than 46mm  (max. length of Aeolidiella spp. in G.B. & Ireland).
- Distance between oral tentacles at base about 3 times thickness of tentacle base .
- Eyes at base of rhinophores not  visible through opaque pigment.
- Rhinophores and dorsum; heavily powdered and marked with opaque pigment, so not translucent .
- Lack pale triangular mark in front of rhinophores.
- Extreme maximum length 46mm (A. sanguinea, others shorter).
- Distance between oral tentacles at base about same as thickness of tentacle base.
- Eyes easily visible at base of rhinophores.
- Rhinophores and dorsum translucent, any opaque pigment widely spaced.
Lower shore and sublittoral. On shores with some hard substrate, including outer estuaries to 20 ppt salinity and muddy sand with isolated stones coated with sediment. Will attack and eat many species of Sea anemone, including Actinea equina (EML) and Metridium senile (EML). Simultaneous hermaphrodite. Convoluted white or pink cord of spawn, somewhat resembling a coiled spring, attached spirally to substrate in January - August . Veliger larvae live in plankton before metamorphosis. Small juveniles not often found on shore; may spend early life sublittorally.
In common with other Aeolidiidae, when attacked, A. papillosa releases stored nematocysts (ejectile toxic stings acquired from prey) from the tips of its cerata. In the case of A. papillosa, the largest European aeolid, the effect is powerful enough in a dog's mouth to make it drop the slug.