To differentiate A. depilans from the smaller, much commoner and widespread (in Britain) A. punctata and from the bigger, rare, in Britain, A. fasciata, see detailed account at https://flic.kr/s/aHskuaWLEz
Aplysia depilans Gmelin, 1791.
1.Parapodial lobes unite at their posterior. Area enclosed by them is close to posterior of animal (fig.3 in Grigg, 1949 http://plymsea.ac.uk/1391/ )
2.Length frequently 120mm or more, up to 300mm.
3.In Britain (2015), confined to south west England; breeds in Cornwall.
The map provided here shows the distribution of the species based on Conchological Society data.