The Snail

Feb 15, 2009 Features / Columnists, Interesting Creatures in Guyana

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The word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled shells in the adult stage.

When the word snail is used in a general sense it includes sea snails, land snails and freshwater snails.
However those lacking a shell or having only a very small one are usually called slugs. Snails that have a broadly conical shell that is not coiled or appears not to be coiled are often known as limpets.

The class of Gastropoda (the snails and the slugs) is second only to insects in terms of total number of species. However, they are regarded as extraordinarily diverse in habitat, form, behaviour and anatomy. As a result what may be true of one snail species may not be true of another.

Snails can be found in a wide range of environments from ditches and deserts to the abyssal depths of the sea. Although most people are familiar with terrestrial snails, land snails are in the minority. However, it is said that marine snails have much greater diversity and a greater biomass since they are in the majority.
Numerous kinds can be found in fresh water and even brackish water.

Many snails are said to be herbivorous though a few land species and many marine species are omnivores or predatory carnivores.

Snails which respire using a lung belong to the group Pulmonata while those with gills form the paraphyletic group. In other words, snails with gills are divided into a number of taxonomic groups that are not very closely related. Snails with lungs and with gills have diversified widely enough over geological time that a few species with gills can be found on land, numerous species with a lung can be found in freshwater and a few species with lungs can be found in the sea.

Most snails move by gliding along on their muscular foot, which is lubricated with mucus and covered with epithelial cilia. This motion is powered by succeeding waves of muscular contraction which move down the ventral of the foot. This muscular action is clearly visible when a snail is crawling on the glass of a window or aquarium. Snails move at a proverbially low speed of one millimetre per second.

They produce mucus to aid locomotion by reducing friction, and the mucus also helps reduce the snail’s risk of mechanical injury from sharp objects. This means that they can ‘walk’ over sharp objects like razors without being injured. Snails also have a mantle, a specialised layer of tissue which covers all of the internal organs as they are grouped together in the visceral mass, and the mantle also extends outward in flaps, which reach to the edge of the shell and in some cases can cover the shell, and which are partially retractable. The mantle is attached to the shell and creates it by secretion.

When retracted into their shells, many snails with gills (including many marine, some freshwater and some terrestrial species) are able to protect themselves with a door-like anatomical structure called an operculum. The operculum of some sea snails has a pleasant scent when burned, so it is sometimes used as an ingredient in incense.

Snails range greatly in size and measure up to 30 centimetre (cm), have a diameter of up to 15 cm and a mass of over 600 grammes.

It is said that most snail shells are right-handed, meaning that if the shell is held with the apex (the tip, or the juvenile whorls) pointing towards the observer, the spiral proceeds in a clockwise direction from the apex to the opening. When the animal reaches full adult size, many species of snails build a thickened lip around the opening of the shell. At this point the animal stops growing, and begins reproducing.

The shells of snails and other molluscs, and some snail egg casings, are primarily made up of calcium carbonate. Because of this, molluscs need calcium in their diet and environment to produce a strong shell. A lack of calcium, or low pH in their surroundings, can result in thin, cracked, or perforated shells. Usually a snail can repair damage to its shell over time if its living conditions improve, but severe damage can be fatal.

Their diet which could also help to improve their health can include leaves, stems, soft bark, fruit, vegetables and algae. Some species can cause damage to agricultural crops and garden plants, and are therefore often regarded as pests. Aquatic snails eat other varieties of food such as plankton, algae, plants, and other microscopic organisms that live underwater.

Most snails bear one or two pairs of tentacles on their heads. In most land snails the eyes are carried on the first (upper) set of tentacles (called ommatophores or more informally ‘eye stalks’) which are usually roughly 75 per cent of the width of the eyes. The second (lower) set of tentacles act as olfactory organs. Both sets of tentacles are retractable in land snails. The eyes of most marine and freshwater snails are found at the base of the first set of tentacles.

It has been observed that snails are able to break up their food using the radula, which is a chitinous structure containing microscopic hooks called cuticulae. With this the snail scrapes at food, which is then transferred to the digestive tract. This is why, in a quiet setting, a large land snail can be heard ‘crunching’ its food as the radula is tearing away at what it is eating.

The cerebral ganglia of the snail form a primitive brain divided into four sections. This structure is very much simpler than the brains of mammals, reptiles and birds, but nonetheless, snails are capable of associative learning.

The lifespan of snails varies from species to species. In the wild, some snails live around five to seven years and Helix snails live about two to three years while there are those that live only a year or so.
Most deaths are due to predators or parasites. In captivity, their lifespan is much longer, ranging from 10 to 15 years for most species. On occasions, snails have lived beyond this lifespan even up to 30 years.

Land snails have many natural predators, including members of all major vertebrate groups, decollate snails, ground beetles, leeches, and even the predatory caterpillar Hyposmocoma molluscivora. The Botia family of freshwater fish also feed on freshwater snails by sucking them out of their shells.

In the pulmonate marsh snail, Succinea putris, there is a parasitic flatworm, Leucochloridium paradoxum, which prevents the snail from retracting its enlarged and parasitized eye stalk, which thus makes the snail much more likely to be eaten by a bird, the final host of the worm.

Humans also pose great dangers to snails in the wild. Pollution and habitat destruction have caused the extinction of a number of snail species in recent years.

Additional they have been eaten for thousands of years. Archaeological evidence of snail consumption is especially abundant in Capsian sites in North Africa but is also found throughout the Mediterranean region in archaeological sites dating between 12,000 and 6,000 years ago.

It should be noted that wild-caught land snails that are undercooked can harbour a parasite (Angiostrongylus cantonensis) that may cause a rare kind of meningitis.
(Source: Wikipedia – The Free Online Encyclopedia)

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