Mystery from Sea of Cortez is Echiuran

Dear Cory,
Thanks for your rapid reply. This mystery is definitely off topic, but I try and steer non-opisthobranch enquirers in the right direction - though in this case perhaps not quite the right direction. My experience with echiurans is largely based on a 20cm long sand-dwelling burrower in New Zealand with a very short proboscis, quite different from these crevice-dwelling tropical forms.

Rick,
the Echiura are another phylum of wormlike animals which have a feeding proboscis which can, in some species, extend out a considerable distance from the head end of the body. In many species the proboscis is bilobed. Cory is suggesting, that your animal is an echiuran, and what you can see is the proboscis extending out from a crevice in the lower right of the photo and ending in two fatter lobes at the top left. The body is hidden. My suggestion was that the fat bit at the top was a nemertean worm and the long thin bit was its proboscis.

I am sure Cory is correct but one way to check would be to give it a gentle prod and see which way it retracts. If its an echiuran it should retract or partially retract down a crevice in the bottom right. Although this is not a 'slug' please let us know what happens.
Cheers,
Bill Rudman