The World of Snails

The classification system was devised by swedish naturalist Carl Linneaus, he had specimens sent to him from all over the world, he named over 10,000 species.

Every living organism is classified into groups according to shared characterisitcs. The latin name of a species is made up of the genus name and the species name. It should be italicized. The name in brackets is the scientist who named the species and ordered it into the groups of classification.

For example: Achatina achatina (Linnaeus, 1758)

Kingdom: Animalia - This contains all the animals of the Earth

Phylum: Mollusca - This group contains animals with no skeleton, but usually a shell to protect their soft and vunerable body.

Class: Gastropoda -  All snails, slugs and sea-hares. These are usually small animals with a long body, tentacles and sometimes a shell.

Subclass: Pulmonata - snails and slugs that breathe through a lung.

Order: Stylommatophora - This group contains all the land gastropods that have eyes on retractable stalks.

Superfamily: Achatinoidea

Family: Achatinidae

Genus: Achatina - Large African snails with a slender shell, a pointed apex and which lay many small eggs.

Species: achatina - The giant tiger snail, the largest species in the world. An orange shell with black stripes, and dark grey skin. Grows to 200mm.

The shell is an armoured shield for the snail to hide against predators. It consists of many layers. An outer layer called the periostracum, this is usually coloured to give the snail some sort of camouflage. A thick inner layer called the ostracum, and another inner layer called the hypostracum. The shell is secreted by a layer of flesh called the mantle, which is underneath the aperture.The shell is connected to the body through strong muscle which is attached to the
columella. By contracting this muscle, the snail withdraws into its shell.

The thickness of the shell is in relation to how much calcium is available. Snails whose diet does not have much calcium usually have thin shells, which are fragile and could break. So most snails live in habitats with calcareous soils so they can build stronger shells. Most snails are dextral, which means they coil to the right. Some species e.g. Archachatina bicarinata are sinistral, meaning the shell coils to the left.  When the snail is born, the shell it has is called a protoconch, from this whorls coil downwards or around this. The edge of the shell around the mantle is called the lip, this is sometimes folded out when the snail is fully grown. Shells come in many shapes. They can be spherical (Cepaea, Helix), Discoidal (Discus), Conical (Obeliscus, Columna).

For anyone who doesn't know - a snail cannot survive without its shell. This is because the organs beneath are delicate and will dry out.

There are three subclasses of snails, Pulmonata, Prosobranchia, and Opisthobranchia. Prosobranchs (or operculates) have an operculum on the back of the foot. when the snail contracts into the shell, the operculum seals it. Sea and freshwater prosobranchs have a gill to breath, but this is lost in land dwelling snails. They only have 2 tentacles, with an eye at the base of each. They always have male and female seperately. Sometimes shell characteristics can show this.

The body consists of three parts: The foot, the head, and the visceral hump.

The head contains four tentacles. The lower two are to feel objects around them, the higher 2 have eyes at the top, these are very basic. Below the bottom two tentacles, there is the radula, a rough tongue which the snail scrapes the food with to eat it. On the right side of the snail's body close to the head are the genitals, a penis and a vagina. In sexually mature snails this can be seen as a white dot. The mating ritual takes a long time, they circle each other, then lean their heads together. The sexual organs are inside the visceral hump. penis, testes, sperm, ovaries, eggs, an oviduct and a pouch or receptacle for storing sperm of another snail. Before mating, a love dart is ejected from the snails genital pore behind its head, this is covered in mucus and makes it easier for the sperm to travel. Then the mating follows. Usually this takes 3-8 hours. With oviparous snails, some time later the eggs are laid (through the genital opening near the head), and take time to hatch according to size. Small eggs can take a few days to hatch (Achatina fulica) wheras large eggs can take weeks to hatch (Archachatina marginata). The baby snails crawl out and eat their shells. With ovoviparous snails (such as Achatina iredalei), the eggs stay inside the parent until they hatch, and then come out the parent's body through the genital opening. There is no parental care, the babies are left to fend for themselves, many may die

The foot is basically a very large muscle which is used for locomotion. By contracting and relaxing muscles in the foot, they can glide slowly along. Mucus glands in the foot help the snail to glide. The fastest snail is Helix aspersa, capable of going at 20 metres and hour.

The Visceral hump contains most of the organs. Snails have a simple organ system, basic organs for breathing, digestion and reproduction. The blood is colourless and is pumped by a heart with 2 chambers. The lung lies above the mantle, with a respitory pore for breathing. The anus lies next to this. Food is transported through the oesophogus to the stomach and then to the intestines. Snail biology is little understood, and there is much variation in the snail species.

There is huge variation in size:

 Punctum pygmaeum, shown here with millimetre scale [picture: Francisco Welter Schultes]

  Archachatina marginata on hand [pictures: Emma Walton]

Most snails are herbivores, eating plants, fruits, vegetables, fungi and algae. They are a great pest to gardeners, who use slug pellets and other deterents to get rid of them. Slug pellets are especially cruel as they dissolve the snail. Snails have wild predators too, songthrushes bash the the snail on a rock to remove the shell. Other predators include rats, hedgehogs and moles.

  Songthrush eating a snail (Cepaea nemoralis)

Snails in temperate climates such as Britain hibernate from October - April, they dislike cold weather. They seal a layer of dried mucus around the aperture to keep moisture. Snails like wet weather best because it helps them create mucus, in hot weather they can dry out. After a heavy rainfall many snails litter the garden finding food. Sometimes snails go into a summer hibernation in a drought, this is called Aestivation. In tropical climates such as rainforests snails are active all year round.

Snails can have many physical abnormalities:

Abnormal number of tentacles:

 Achatina fulica with three ommatophores (eye stalks) and four eyes.

 Helix pomatia with one ommatophore.

Albinism:

 Albino-bodied Achatina reticulata on right, normal coloured specimen on left.

Coiling of the shell reversed:

 Sinistral Cepaea hortensis on right, with normal dextral specimen.

Most snail species have shells that are dextral: the shell coils to the right. In rare cases the shell coils the opposite way to normal. In species where a sinistral shell is the norm e.g. Vertigo angustior, dextral shells are rare. This condition is very rare, it usually occurs in less than 1 in 100,000 specimens. A few species (e.g. Caracolus marginella and others in the Camaenidae family) have a similar proportion of both dextral and sinistral forms.