Freshwater snails

through 90 - 180^o bringing the mantle cavity and anus to the side or over the head region, (see the very simplified diagram above right which shows, from left to right, the progression of torsion).

Shell coiling in snails

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 in snails

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.

Snail body plan

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

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.

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.

Genus Limnea - the common pond snails.

All have brownish shells, and flatish, triangular tentacles, with an eye at the base (see above). Above left is Limnaea truncatula, aka Galba truncata, the dwarf pond snail. When fully grown it is around 6 mm wide and 7 mm long, but some specimens can reach 12 mm, and dextrally coiled. It can live for some time out of water. It is the intermediary host of the common sheep liver fluke, Distomum hepaticum, or Fasciola hepaticum. Inside the body of the snail the fluke passes through several larval stages. The final larval stage called a cercaria, bores its way out of the snail's body and climbs up a plant, and if eaten by a sheep, finds its way to the liver. It is found in ponds, ditches and wet pastures.

Above right is Limnea auricularia, the ear-shaped snail or ear pond snail . It is fairly common in larger water bodies. It is dextrally coiled, the last whorl of the shell and the opening are large and ear-shaped. It breeds early in the year and dies afterwards, so few adults are seen after June. It is found in calcareous lowland lakes and slow, weedy rivers.

Family Planorbiidae.

There are species from this family found all over the world. Some are found in habitats which dry out temporarily.

Genus Planorbis - the trumpet or ramshorn snails

The shells are coiled in a flat spiral, and the body is red as their blood has haemoglobin. The tentacles are long and thin, and the foot is short, ending in a point.

Above left is Planorbis corneus, the great ramshorn, it is the largest British species in the genus. It has a dark, red/brown shell which can reach 3.5 cm in diameter. The shell is brown - brown-olive. It is found in calcareous ponds, lakes and slow-flowing rivers. It lays around 60 eggs at a time in a gelatinous capsule which is usually fixed to a plant. There are albino varieties with transparent shells and bodies.

On the right and left is the freshwater ramshorn snail, Planorbis planorbis . This snail has a small head and foot, and the tentacles are relatively long. Unusually it has red blood for storing oxygen. When fully grown it has 5 - 6 whorls. On the left you can see it has a keel which is closer to the undersurface of the shell. The shell is light brown and 14 - 17 mm in diameter when fully grown. It is found in overgrown, stagnant water, muddy pools, creeks, ditches and swamps. It can withstand periodic dry spells. It is found in Europe, Northern Africa across to Syria, the Caucasus and east to

Lake Baikal.

On the left are Planorbis planorbis eggs. The darker pink/orange dot within the egg is the embryonic snail itself.

Above is Paludestrina jenkinsi, Jenkins's spire shell. It is dextrally coiled with 5.5 whorls when fully grown, and 4 - 5 mm in length. It is back/brown in colour. Originally it was found in salty water only, but now it is to be found in fresh water. Females give birth viviparously to 20 - 30 young. They are parthenogenic.

Above is Paludina viviparia, the freshwater winkle. It is parthenogenic and found in slow-running water. As its name suggests it produces live young (viviparous); about 50 at a time. It has a brown shell with 3 dark bands running round each whorl.

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