Polycera quadrilineata (Müller, 1776)

29 August 1993.
At the Battery, Dunbar on the east coast.
At that time of the year these animals are present in their hundreds feeding on the bryazoans growing on the kelp.

for more information about this species go to one of the links below
OPK - Opistobranquis nudibranchs of the british isles sea slug forum