Periwinkles can be described as small sea snails. They are marine gastropod mollusks that are commonly found on rocky shores and tidal areas. These snails have spiral-shaped shells that are usually no more than an inch or two in length. They use a muscular foot to cling to rocks and other surfaces, and they graze on algae and other small organisms. Despite their small size, periwinkles play an important role in their ecosystems as a food source for many predators, including birds, crabs, and fish. So, to summarize, periwinkles are small sea snails that live in rocky intertidal zones and are an important part of the marine food web.