Observing a living snail crawling its way, it is easy to see the difference between the soft, flexible living body covered in mucus and the hard, lifeless shell.
What is visible outside of the shell, can be divided into two functional parts: The larger part, the snail's foot, is flattened below to form a sole, on which the snail moves and the front part is called head.On the snail's back there is the visceral hump, which contains vital internal organs, such as a large part of the digestive system. It is surrounded by a protective tissue layer called the mantle (pallium). There also a large opening can be seen, the respiratory hole, opening to the mantle cavity beneath.
Body parts and organs of a Roman snail (click here)
All that is visible of a snail's body is covered in mucus or slime. Many gland cells are dispersed over a snail's body surface and constantly produce mucus.
A terrestrial snail's slime not only protects it against drying out, but also against smaller attackers, such as ants. Then the snail produces large amounts of watery slime bubbled up with air blown from the mantle cavity. Some snails also can make a grunting or hissing sound doing so.
A snail's head is where most of its sense organs are concentrated. When it moves, the snail continuously orientates in its vicinity by moving around its tentacles. The shorter, lower pair of tentacles is pointed to the ground and used as feelers. Both tentacle pairs are densely set with sensory cells of smell and taste. The lips, which are below the short tentacle pair, also are densely set with taste cells.
A Roman snail's foot is specialised in a crawling motion. That is why its underside, the foot sole, is flat to make a gliding surface. The snail moves by forming a wave (visible from below as a dark transversal band).
The suction of a snail's foot can clearly be observed trying to pick it up, especially from a smooth surface. A large mucus gland produces a slime bed the snail glides on and that it leaves behind as slime trace.
Mantle and Visceral Hump
A part of a snail's body never leaves the shell. Those body parts of a snail are sack-shaped. In this body part's interior there are numerous internal organs of the. Because it contains internal organs, this part of a snail's body is called visceral sac or visceral hump.
The mantle fold in the shell mouth is a special adaptation of terrestrial snails. The respiration takes place in the mantle cavity beneath the respiratory hole. The mantle, however, not only reduces evaporation, it also builds the shell. Gland cells on the mantle rim folds produce a calcareous secretion, which hardens to form the main shell layer.
It was the capability to breathe oxygen from the air that finally enabled today's terrestrial molluscs' ancestors to move on dry earth. The respiration takes place in the pallial cavity. That is why it is also called the snail's respiratory cavity. Gas exchange takes place at the cavity's roof. There the tissue is particularly thin, so oxygen (O2) diffuses into the blood fluid circulating there.
A Roman snail's heart is located in the heart-bag (pericardium) at the upper mantle rim. A Roman snail's heartbeat frequency depends very much on the snail's body temperature and activity. It ranges from 70 to 80 beats per minute in an awake and active snail to about 5 beats per minute during aestivation, but especially during hibernation. Finally, a snail's also gives form and firmness to the snail's soft body, which, the snail being a mollusc, lacks a skeleton. As it is the blood responsible for the body's firmness, it is also usually referred to as a hydroskeleton. Additionally the blood fluid of a snail is used to stretch out withdrawn parts of the body by hydraulic pressure.
After a snail has taken in its food, rasping it into tiny little bits with its radula, the food disappears in the snail's gullet to be digested.
The first steps of digestion already take place in the snail's mouth. Here the snail changes the food into pulp that can easier be transported into the gullet.
At its rear and the stomach or crop is a blind sack, in which the food pulp is dammed. Here the main part of digestion takes place. The digestive fluids are produced by the main digestive gland. Another function of the main digestive gland is to extract calcium carbonate from the food and to store it. The snail needs this calcium carbonate for her own shell, to make egg shells and also the hibernation lid is made with this.
The function of organs in a Roman snail's organism is controlled by the nervous system. This system is In Helix pomatia, this evolution has gone so far, that the single ganglia partly are molten, like is displayed in the illustration. So it may be said there is a clear tendency towards development of a brain (cerebralisation) in Roman snails.
Senses and Sense Organs
Terrestrial pulmonate snails adaptation to life on land could only be so successful because next to respiration, reproduction and development also senses and sense organs of terrestrial snails have widely adapted to this new form of life.
The Roman snail only has two kinds of those sense organs in the stricter sense of the word: Eyes at the tip of the longer tentacles and statocysts, fluid-filled capsules near the circumesophageal ring, serving as equilibrium organs. The Roman snail's remaining do not work by sensual organs but by single, and rather disperse, sense cells. Those may be concentrated mainly on the head, like fort he senses of smell and taste, or dispersed in various densities, over the whole body surface.
Sense of Light
The two longer tentacles of a Roman snail are thickened at their ends and there each carries one eye. It is a quite intricately structured eye.
One has to assume that Roman snails, by the help of the optical pigment rhodopsin, can only see black and white.
As it is not that efficient as a fully developed lens eye, Roman snails are probably quite short-sighted. Their highly developed lens eyes give them information on light as well as contrast.
There are also further light sense cells in a snail's body: They are dispersed over the body surface. This provides the snail with the ability to the so-called shadow reflex: A shadow suddenly falling over the snail will cause it to withdraw into its shell, even if it cannot yet see, what exactly is causing the shadow.
Sense of Touch
All of a snail's body is susceptible to touch. Should the snail touch an obstruction, it will at once withdraw the respective body parts with fast-acting retractor muscles. In the extreme, the main retractor or columellar muscle, can withdraw all of a snail's body into the shell. While the snail crawls over the obstruction, the foot sole keeps feeling the surface and adapts to its form.
Sense of Equilibrium
Where their path leads them, snails know only few limits. They are able to crawl along the thinnest of branches backside-down. Turned on its back, the snail will try to return to its normal state by moving its foot. The assumption is allowed, that a snail, to be able to achieve those things, must be aware of its position in space. That is possible because of the snail's organs of equilibrium, the statocysts.
Senses of Taste and Smell
It is well known, that snails and especially slugs are able to find food in considerable distance. They move there in a decisive manner, even without being able to see the food source. In experiments it could be proved, that Roman snails move towards artificial sources of smell, as they do towards natural ones. Mainly in the lips, but also in the smaller and to a lesser extent in the longer tentacles, olfactory sense cells could be found. Additionally, snails are able to perceive olfactory stimuli with the remainder of their body surface, where sense cells are dispersed.
Senses of Temperature and Humidity
The ability to determine changes in temperature and humidity in the surrounding environment is of vital importance to a Roman snail. Snails will actively avoid dry areas and prefer humid cool ones. This fact is also easily seen in gardens, where slugs approach preferably freshly watered places first.
Roman Snail producing Mucus
Head of a Roman Snail
Foot of a Roman Snail
- Picture: Weichtiere.at -
Mantle with Respiratory Hole
- Picture: Weichtiere.at -
Fast-acting Retractor Muscles
Antique scientific Drawing
of a Roman Snail
Eye of a Roman Snail
Roman Snails walk the Line