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Gastropoda (snails, marine and freshwater snails, slugs, etc)
The gastropoda are a Class in the phylum Mollusca, for more pages in the phylum click below left.
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These 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, with 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" some have become asymmetrical. 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 right which shows, from left to right, the progression of torsion). |
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| 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. 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. 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). |
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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. 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, which when fully grown will reach a height of 25-35 mm and a width of 25-40 mm.on the right, which shows two snails 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. The dart is mainly calcium carbonate, and its function is not known. It may arouse the other snail, heighten excitement, or inhibit further matings. |
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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 above. 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 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 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. |
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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 this tiny snail, 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. The giant African land snail can be as much as 30 cm long. Originally it was found only in East Africa, |
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but it has been introduced to most other parts of the world. in certain areas of southern Asia it has become an agricultural pest.
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Above right is a small terrestrial snail Cochlicella acuta. I found many of them on a white wall in Rhodes. It is around 10 mm long and 3-4 mm wide. Some books say that they cannot be found as far south as the Mediterranean, but this is not the case. It is very similar to the slightly stouter Cochlicella barbara. On the left and right is one of the most commonly found snails in the garden, Cepaea nemoralis. The light colour varies considerably from white and yellow through to pink and brown. This is the snail often used in schools when pupils mark it to study movement. The scale shown on the right is in millimetres. |
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| Clausiliidae family. Commonly known as door snails. The shells in this family are usually coiled sinistrally (to the left), are sometimes ribbed and narrow. The opening is small and there are usually teeth, folds and projections. There are over 150 described species in Europe. Most live in forests and among rocks. They hide in crevices, under bark and in heaps of fallen leaves. At night, and when it is wet they crawl out onto rocks or up tree trunk in search of their food - algae and lichens. Cochlodina laminata (aka Marpessa laminata), the painted door snail on the right is common throughout Europe and is associated with birch trees. It lays its eggs in August and September. The youngs take 2 years to reach maturity, and it is only then that the characteristic teeth at the shell opening develop. The shell has fine, irregular grooves, and is yellow-brown, reddish-brown, height 15- 17 mm, width 4 mm. It is found in woods beside trunks and in bark. |  |
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