I have. The cocoon is soft and white when the adult worm 'wriggles out of it' and only turns hard and yellow later.

Thanks for the reply cath. Does the parent, when it 'wriggles' out of the cocoon, move quickly? Or is it a slower process. I have no idea what the cocoon looks like while it is still attached to the parent. Is it visible then?
Janet T

The cocoon looks like part of the worm, so you won't really notice it. It's really just a ring of mucous, which as long as it's still attached to the worm, stays moist and translucent, tending toward white just before it's cast off. Then it hardens when it's no longer on the worm.

Thanks again cath! That makes perfect sense. I have seen the cocoons as they are still attached to the worms per your desciption. I just haven't seen them come off the worms.
J