Unidentified tentacles

Can anyone tell me what these tentacles belong to? I've spotted these several times while diving at the Dimaniyat Islands in the Indian Ocean off the Omani coast. All that I can see are this T-shaped tentacle which only moves if something touches it. I have never been able to see what it is attached to as the end is always hidden under the coral. No one I've asked has been able to help.

i believe they are some sort of snail actually. Leslie should know

I think it's some kind of Echiura and the part that is visible is its proboscis, the rest of the worm is hidden

Asbjorn

Asbjorn gets the gold star today. Echiura is a group of small soft-bodied worms that many consider to be a subgroup of Annelida, the phylum that contains polychaetes, earthworms, & leeches. Usually the proboscis is simple but in the family Bonellidae the proboscis is T-shaped. Nearly all echiurans feed by extending the proboscis (which is highly contractible and attached outside the mouth) then collecting bits of detritus & tiny animals which are moved back to the mouth by synchronized tiny hairs called cilia.

Thank you Asbjorn for identify this for me and also Leslie for your full and comprehensive post. Next time I spot one I'll be able to impress my buddies with my new found knowledge. Now that you have named the species for me here's some other fascinating facts I found:

From The Marine Fauna Gallery of Norway http://www.seawater.no/fauna/Polseormer/viridis.htm

You will only encounter the females, as the males are of microscopic size and live inside the females.

and from Wickipedia http://en.wikipedia.org/wiki/Bonellia_viridis:

The planktonic, free-swimming Bonellia larvae are initially sexually undifferentiated. Larvae which land on unoccupied sea-floor mature, over the period of years, into adult females. But most larvae come in contact with the bonellin in the skin of an adult female -- its body or its roving, bonellin-rich proboscis -- and are masculinised by this exposure. The chemical causes these larvae to develop into the tiny males, which cling to the female's body or are sucked inside it by the feeding tube, to spend the remainder of their lives as parasites inside the female's genital sac, producing sperm to fertilize her eggs and reliant on their host for all other needs.

Typical males!