Mystery pond creature

This creature has been regularly caught in our pond recently and I cannot find it in the ID book.

It has no legs as far as I can see, so i doubt its a caddisfly larva.

It appears to be a worm inside a jelly-like case. Often you can see the worm through its case, and it sometimes extends some of its body out of the case. It also wiggles a lot. The clear case can have a thin green film, which appears to be algae.

ID suggestions will be much appreciated!

No idea, myself, but you will have more chance of help by posting it on the observations pages. The forums are not much visited.