Not all a Terebellids' tentacles need be out at one time. This looks
just like a Terebellid. There is a relationship between tentacle length
and worm length, but it does not follow that a worm need be longer than
its tentacles. But, Terebellids can get quite large-10-15 cm.
Geoff Read wrote:
> Actually, I have another suggestion. If those are mainly feeding trace grooves rather than tentacles (the photo size is too small to tell, and there is high contrast, but I am suspicious) it could be an echiuran burrow in the middle. They have amazingly long reach with a single proboscis. Yes, terebellids get very big - but don't they have many more tentacles than that?