Snails are one of the most common types of mollusks found in the world's oceans. Most feed on algae and small invertebrates, but some carnivorous species will actually capture small fish by stinging them and injecting a powerful toxin. Some are even poisonous enough to kill a human. The snails have a hard shell formed of calcium carbonate that is secreted by the animal as it grows. This gives the shell a spiral pattern.