Typically, snails coil as they grow. The exact shape and characteristics of the coil are known to be a combination of genetic and environmental factors, depending on the snail. There is an interesting story involving snails and the young Jean Piaget. Piaget is famous for his work in psychology, but before that, when he was quite young, he worked with birds and mollusks. His work was published and otherwise disseminated and experts in these areas, unaware that he was a teenager, assumed he was an adult expert on natural history.