Blue Bivalve shell with long worm thing!

[sally]

A log on the shore line in Wales, UK was covered in pale blue bivalve shells, with a dark blue line around, and a worm type thing attached. The "worm" was about 6-8" and about 1/2" thick, and the shell was attached near one end, with a sort of "tail" inside, that sometimes unfurled to reveal a fin shape end. The whole log was covered completely. Anybody know what they might have been, as I cannot find any reference to them anywhere? Unfortunately I have not got a picture.