My vote, Sweat Potato Sea Cucumber, (Caudina arenicola). The shell to the left looks like the inside of a mussel shell. So in comparison to that, the crab and the size of the gravel, I'm guessing that this is fairly large. Peanut worms tend to have a longitudinal and radial pattern to them that I'm not seeing in the picture (then again my eyes ain't young no more;Oldman).