Grey sea slug (Aeolidia papillosa), from the Clyde - April 2011

A surprisingly big 4 inch long grey slug from the Clyde - sometimes called the shag rug nudibranch.
The grey sea slug / nudibranch (Aeolidia papillosa)

Sometimes aptly named the shag rug nudibranch. This one at Wemyss Bay was large at around 4 inches long.

Nikon D90, Nikkor 60mm