Gastropoda (snails, slugs, etc.)

Introduction to the Class Gastropoda

Class Gastropoda are the snails, slugs, limpets, conches, sea butterflies and sea hares. This is a very diverse Class, and the most abundant and widespread class of molluscs, they can be found in sea and fresh water as well as land. There are about 77 000 described living species and 15,000 fossil species, and can range in size from just a few mm to 600 mm, or even to 1 metre in the case of the sea hare Aplysia sp., some fossil species are 2 m long.

They are bilaterally symmetrical, but because of torsion - see below, some have become asymmetrical.

Torsion

Torsion occurs at the larval stage and involves the visceral hump. The two foot retractor muscles develop at different rates. This, along with the uneven secretion rate of the shell from the mantle, twists the visceral mass through 90 - 180o bringing the mantle cavity and anus to the side or over the head region, (see the very simplified diagram on the right which shows, from left to right, the progression of torsion).

The gastropod body

Gastropods have a muscular foot, distinct head region, radula (see main Mollusc 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.

Gastropoda 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.

diagram showing the steps of torsion in snailssnail showing main body parts

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.

Slugs and snails have two pairs of antennae/tentacles. At the end of the longer pair are the eyes. The shorter pair are chemoreceptors, this can be seen in Helix aspersa, now known as Cornu aspersum, the common garden snail below right, which shows two snails mating, which when fully grown will reach a height of 25-35 mm and a width of 25-40 mm.

Identifying the direction of coiling (torsion)

The drawing on the right shows how to identify the direction of coiling or torsion. Torsion can be either dextral (to the right) or sinistral (to the left).

marine and freshwater snails, slugs

Snail shell direction of coiling (torsion), dextral and sinistral

Snail mating

At a prelude to mating the two snails touch antennae. Then if both are agreeable there follows a period of fondling and touching that culminates in both snails stretching out with the base of their foot sole to sole.

Next they retract back into their shells a little and one shoots a chalky dart into the other as is seen in the image below left.

Helix aspersa, the common garden snail mating

The dart is mainly calcium carbonate, and its function is not known. It may arouse the other snail, heighten excitement, or inhibit further matings.

Both partners fire darts before copulation can take place, but it seems that the individual who fires the first dart controls the timing on events.

It is only after this that copulation takes place, see the photograph on the left and below.

Most terrestrial slugs and snails are hermaphrodite, so any two individuals of the same species can mate - handy for such slow-moving animals.

During copulation each snail transfers a spermatophore to the other. See the photograph below left showing the partial eversion of the genitalia through the genital pore. The vagina and penis are thrust out through the genital pore to make contact with the partner's vagina and penis.

common garden snails passing a chalk dart as a prelude to copulation

As you can see in the diagram above the genital pore is located behind the head. Copulation can last several hours. The two photographs above and below were taken once copulation was well underway and three hours later the snails were still attached to each other.

When the eggs are ready to be laid they too must pass through the genital pore.

Helix aspersa, the common garden snail, genetalia during mating

The genital pore is located on the right side of snails whose shell turns to the right, and the left side of those whose shell turns to the left.

The eggs are usually deposited in a gelatinous mass in shallow burrows or the undersides of stones.

The result of the mating above was the tiny snail on the right, and many more like it. It measured just a few millimetres across, but I'm sure will grow as it eats its way through my plants.

Helix aspersa baby common garden snail

Cornu aspersum or as it was previously known, Helix aspersa is in the photographs above and left, is native to western Europe and around the Mediterranean, but has been introduced world wide in temperate and sub-tropical areas.

Fully grown adults are 25 - 40 mm in diameter, and 25 - 35 mm in height. There is great variation in the shade of brown and the flecks of colour.

Mating is mostly seen in early summer. About 2 weeks after mating about 80 spherical, pearly white eggs about 4 mm in diameter are laid. A healthy snail can lay around 6 batches a year. The young take 2 years to reach maturity. Their life span is 5 - 10 years, but individuals have been known to reach 15 years of age!

They are mainly nocturnal, but are active during the day in wet conditions. This species has a strong homing instinct, so throwing yours into your neighbour's garden won't work unless you throw them far enough.

It hibernates, mainly in the soil, in cold winters.

 
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