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!