It extended a striped neck.
I think. . . Some kind of worm.
Living in a 1/2" shell, which it could move around; actively feeding in nearby sand.
This worm, which I think is Themiste pyroides (Burrowing Peanut Worm), was between some rocks with what I think is Paranemertes peregrina (Mud Nemertean) and some Green Ribbon Worms, as you can see in the second photo.
Cream with brown spots, found in collected mud under/between rocks. ID by Assoc Prof Keith Probert.
Under rock in sand at Botanical Beach, Juan de Fuca Provincial Park, BC
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.