There are also many species of snails that spend their entire lives in the water, such as the sea butterflies, which are called Pteropods. Few species of sea butterflies are found in the North Sea. There are sea butterflies with a snail shell (Thecosomata) that can be spiral shaped like the genus Limacina, but also species with a snail shell that is no longer spiral shaped, like the genera Cavolina and Clio. Most species are filterfeeders, and many make a web of mucus to catch particles.
Furthermore there are snails without a snail shell (sea slugs) called sea angels or Gymnosomata. These are often a lot bigger, and they are mostly predators that eat other pteropods.
Free swimming snails move by moving flaps (the “wings”).