Whose eggs on the beach?

Whose eggs on the beach?

Observation - Whose eggs on the beach? - UK and Ireland. Description: I came across masses of these elongated jelly bags (c 2-3inches long) with one end buried in sand with a mass of tiny eggs inside exposed on a a very low spring tide on a wide sandy beach in Wales. Maybe from some kind of gastropod, annelid or nemertine

I came across masses of these elongated jelly bags (c 2-3inches long) with one end buried in sand with a mass of tiny eggs inside exposed on a a very low spring tide on a wide sandy beach in Wales. Maybe from some kind of gastropod, annelid or nemertine worm? They don't match anything I can find pics of exactly but must be of something pretty common on sandy shores from the numbers. (Added later after ID made. This link shows the shell of the adult shelled seaslug, in use by a hermit crab http://www.ispotnature.org/node/286103)