Checkered hairy snail on serpulid worm
Checkered Hairy Snail, Trichotropis cancellata, a kleptoparasite snail
R & B, Browning Pass, British Columbia
From Leslie Harris:
"It steals food particles caught by the worm before the worm can swallow them. Snails that do this grow faster than ones who don't and the worms that are parasitized grow more slowly. Serpula columbiana and several of the big fanworms are the victims."