Molluscs are all non-segmented invertebrates with a body cavity containing organs, along with a head region and a foot region. They usually possess a muscular mantle covering their bodies, which usually secretes a shell. They are an incredibly diverse group with over 50,000 known species, containing everything from the simple sessile filter-feeders, such as the mussel, to the predatory octopus, thought to be the most intelligent of all invertebrates.
According to the University of California Museum of Paleontology, molluscs are an old group of animals, with fossil evidence for their existence dating back over 500 million years. They have a variety of different body plans, from the largely sessile bivalves, such as clams and oysters, to the elongate crawling groups, such as snails and chitons, to the swimming tentacled groups, such as squids and nautili. Most species are marine, with some occurring in fresh water and others on land.
Only snails and slugs are well-represented out of water. They must be kept constantly moist, which limits their distribution. This group, the gastropods, are the most successful group of asymmetrical organisms in existence, having evolved out of the bilateral symmetry typical of other molluscs. The slugs on land are more closely related to land snails than they are to sea slugs.