This insignificant nudibranch (or sea slug) has no common name and is known as Onchidoris bilamellata. Growing to a maximum of 4cms, it feeds entirely on barnacles. They come together in great numbers to mate and lay their eggs in any fairly shallow, rocky area around our coasts. The fan-like arrangement at the rear is a combination of gills and anus. See the separate picture of a mating group.
St. Abbs Marine Reserve, Berwickshire. E. Scotland