Most of the living molluscs in caves are gastropods and many are omnivores – like man they feed on a variety of food types. Many produce chitinase enzymes to digest the chitin in the exoskeletons of insects e.g. Cellar snail Oxychilus cellarius.
The Glass snails, in the genus Oxychilus, are the snails most commonly recorded from Devon caves, including Oxychilus draparnaudi, O. navarricus (was O. Helveticus), O. cellarius and O. alliarius. 

Glass snail, probably Oxychilus draparnaudi, Spider Cave

Other species include the Common Door Snail Clausilia bidentata, the Rounded Snail Discus rotundatus and the Common Garden Snail Cornu aspersum (aka Cryptomphalus aspersus, previously Helix aspersa), along with several other species, all of which are generally found in the threshold zone.  The Great Grey Slug Limax maximus has been recorded from Napp’s Cave near Ifracombe.

Common Garden snail, Cornu aspersum, Spider Cave

The Bivalves (so named because they have two shells (valves) joined together by a hinge) are another group of molluscs found in both fresh and marine waters.  Marine bivalves include creatures like the cockles and razor shells etc. Freshwater bivalves rarely occur in caves, although a population of the Pea Mussel Pisidium personatum exists in the pool at the end of Fairy Hall Quarry Cave.