Many species have yet to be discovered and many recently discovered species are yet to be identified and named. According to some scientists, Mollusc evolution began more than 500 million years ago, during the Cambrian period. Paleontologists use fossil shells to tell what the climate might have been like millions of years ago. This is done by comparing the fossil shells with their living relatives of today by noting where they live such as in warm or cold, and wet or dry climates.
All molluscs possess a fleshy mantle.
Some molluscs use this mantle to produce a shell by absorbing calcium carbonate and other ingredients from their habitat and food and secreting it in an orderly fashion to form a shell house. Molluscs keep adding to this shell and keep it under repair for its entire life. Slugs leave a slime trail behind them as they move. This makes it more difficult to eat them, as well as giving themselves a coating with an unpleasant taste to potential predators. Their slime helps them fit through a tight squeeze.