Gastropods with coiled shells can be right-handed (dextral), or left-handed (sinistral). Dexterally coiled shells are more common, and the coiling direction is under genetic control.

Identifying the direction of coiling. Hold the shell with the opening facing you, and the pointed end pointing up. If the opening is on the right and the curve goes to the left it is dextral, as in Cepaea nemoralis. The opposite is sinistral.

Gastropods have a muscular foot, distinct head region, radula (see main page for diagrams), one pair of eyes and sensory tentacles.

Slugs and snails reduce surface friction when moving by secreting mucous from the foot. The foot is extended hydraulically by pumping it up with blood. Many have a spirally coiled shell. They are divided into two Sub-classes.

Above is Strombus gigas, the queen conch. It is the largest North American mollusc, with some specimens reaching just over 30 cm. It is found in the Caribbean and surrounding waters usually on sand.

The Sub-class Heterobranchia includes the terrestrial and aquatic snails, and slugs. There is a tendency towards loss of the shell, e.g. in slugs, and hermaphroditism in terrestrial species. Some authorities separate this Sub-class into the Sub-classes Pulmonata (containing most of the terrestrial and freshwater species; they have lungs and are mainly hermaphrodite); and the Sub-class Prosobranchia (containing the marine snails and a few freshwater and terrestrial species). The characteristic used to spilt them is the operculum (a horny lid used to close the opening of the shell). If it has an operculum it is in the subclass Prosobranchia, if it doesn't it is in the Pulmonata. Also Prosobranchia have one pair of tentacles.

On the right is Cassis madagascariensis which is often used by jewellers for carving cameos. It is a possible predator of invasive sea urchins, and is found in tropical and temperate oceans from surface to 1000 m deep.

Above Triton australis

Limpets graze on algae and will return to the same place before low tide. They will eventually form a "scar" on the rock which perfectly fits their shell outline. Abalone are also algae grazers. The series of holes in their shells are used for drawing a current of water to pass over their gills, and for expelling waste. Some species have beautiful iridescent shells which are used for making jewellery.

On the right is Ancylus fluviatilis, the river limpet. It has a thin shell which is slightly hook-shaped. when viewed from the side, and 5 - 8 mm high. It clings to stones in fast-flowing rivers and streams, and occasionally at lake edges. It requires clean, silt free water. The edge of the shell is not hard, and so can deform to fit the irregular shape of stones. It lays eggs, usually 2 - 10 in a round, transparent capsule attached to stones. It is found throughout the U. K.

The Sub-class Opisthobranchia contains all the other Gastropods, e.g. the sea slugs, sea hares and sea butterflies. They are mainly marine, and are mostly found in shallow waters. They show partial detorsion and many have no shell.

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