Common Periwinkle
Littorina littorea
Size: to 1.2"
Habitat: intertidal, on any solid substrate
Notes: vegetarian - feeds on seaweeds
This is another good snail for a cold-water aquarium, as it will constantly clean all surfaces of algae, and also consume any other waste it finds. They are also edible.
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 ...