I took this opportunity to read about the lowly periwinkle and they are quite prevalent creatures! I stopped at the part about how to prepare and eat them…no thanks.
“They are nonnative (meaning they aren’t originally from here). They were brought over from Western Europe to be used as escargot (cooked snail). In other words, they were brought here to be eaten. Some people think they might be invasive (a nonnative that has a negative effect on the local environment) and compete with native periwinkle snails for food and space. The tentacles on their head are sensory organs that are used to see and taste. They have a strong muscular foot (which is the main part of the body that we see outside of the shell) that helps them to slide along the ground and to hold on tight to the rocks when the waves are crashing on them. Periwinkles cannot swim, but they can breathe underwater. They are herbivores (animals that eat plants).” National Park Service http://www.nps.gov