This is the first time I've found Phascolosoma agassizii, or Agassiz's Peanut Worm. It was found under a rock at low tide, and measured between 2 and 3" long, about as wide as a pencil.
Worm-like thing sticking out of a hole in the rock. It withdrew when I disturbed it.
There were literally hundreds, if not thousands, of these on the beach. They created small mounds, many of which had one end of the sea cucumber partially exposed.
Another family found these recently and dug one up. They're definitely peanut worms. The picture of the one in the hand is apparently only a part of the organism.
For the full write-up on this trip, visit: http://keepontrekkin.ryanmarquis.com/2013/06/03/a-may-day-at-bishops-beach/
Morphology of the introvert hooks unknown. We only have this picture the picture to go by.
The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a group containing 144-320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms. Traditionally considered a phylum, they might be a subgroup of phylum Annelida based on recent molecular work.