Mystery Pond invertebrate

Mystery Pond invertebrate

Observation - Mystery Pond invertebrate. Description: Can anyone help identify this invertebrate? It looks like a blob of transparent 'jelly' but there is a tube/ worm inside that wriggles about and occasionally either extends out of the end of the 'jelly' or comes out completely and moves around on its own-

Can anyone help identify this invertebrate? It looks like a blob of transparent 'jelly' but there is a tube/ worm inside that wriggles about and occasionally either extends out of the end of the 'jelly' or comes out completely and moves around on its own- is it some sort of fly larvae? Would be very grateful for any help on this!