Most species move by gliding on the muscular foot, but in a few species- the sea butterflies and heteropods- the foot is a swimming organ, with which the animal moves through open ocean waters. The nudibranchs, or sea slugs, have lost their protective shells over the course of evolution; their sometimes brilliant collation is aposematic; that is, bright colors often serve to warn potential predators of toxicity.
Shelled gastropods have one-piece shells. The only mollusks that live in terrestrial environments- land snails and slugs- are gastrpods. In these terrestrial species, the mantle tissue is modified into a highly vascularized lung.
Gastropods are characterized by the possession of a single (often coiled) shell, although this is lost in some slug groups, and a body that has undergone torsion so that the pallial cavity faces forwards. They have a well-developed head bearing a pair of cephalic tentacles and eyes that are primitively situated near the outer bases of the tentacles. In some taxa the eyes are located on short to long eye stalks.
Most aquatic gastropods are benthic and mainly epifaunal but some are planktonic. A few such as the violet snails (Janthinidae) and the sea lizards (Glaucus) drift on the surface of the ocean where they feed on floating siphonophores, while others (heteropods and Gymnosomata) are active predators swimming in the plankton. Some snails (such as the whelk Syrinx aruanus) reach about 600 mm in length.
Sea-living snails and slugs typically start life as a tiny larva floating in the plankton, and only later metamorphose into a crawling creature. On land and in freshwater, however, miniature versions of the adults usually hatch from eggs.
With their messy trails and taste for greens, garden snails are often considered to be pests whose strong homing instinct makes human control difficult. They are often seen after rain, and leave a tell-tale trail of mucus. Being hermaphrodites, garden snails each have both male and female reproductive organs, but although they can mate with themselves, it's more usual to find a partner.
Many sea slug species also have camouflage colours, which make them disappear in front of their habitat's background. Predators can also be confused by suddenly appearing colours or even light (bioluminescence). But by no means all sea slugs are nudibranchs; shell-less forms are present in all opisthobranch groups, and even in some groups that are not opisthobranchs.
Probably best known among sea slugs are the very colourful forms of nudibranchs (Nudibranchia), the diversity of which keeps on astonishing divers in all parts of the world. Nudibranchs, like all gastropods cannot see colours. But many sea slugs have defensive or even toxic compounds at their disposition, mostly acquired with the food (such as blue algae, Cyanobacteria).
Gastropods, the only mollusks with terrestrial representatives, occur in nearly every habitat type. On land they’re found in wet and dry areas, including deserts; from low to high elevations; from tropical to polar latitudes (as high as there is humic material and leaf litter).
There are over 40,000 species of snail and slug. Most have a coiled shell, and those that do not are usually called slugs. Most snails and slugs have a twisted body, where their back end is twisted up over their head.