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 nad 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
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).
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).
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.
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, 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.
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. 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.
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.
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.