Oyster Drill
Urosalpinx cinera
Size: to 1"
Habitat: subtidal down to 50 ft
Notes: This little snail uses acid and its rasp-like tongue to drill a neat hole in a bivalve or barnacle victim, then sucks out the contents.
These creatures are all of the order Gastropoda - having a single, often coiled, shell, as opposed to the bivalves, which have two matching shells. Most snails are hermaphroditic. Also, most snails have a right-hand twist to the shell, although there are exceptions.
Right:
X-ray image of a Channeled Whelk, showing internal structure.
Nudibranchs are a form of snail that has lost its shell, while Corollas and Sea Butterflies are snails that have abandoned not just their shells, but the snail-like existence entirely, swimming up into the water column as plankton.
More: Snails ...