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 A Roman snail's head with the characteristic four tentacles. [RN] |
A Roman snail (Helix pomatia) crawling its way shows well the characteristic body division into a hard lifeless shell and a soft, living body. Snails, in contrary to numerous other molluscs, possess four clearly distinguishable feelers or tentacles sitting on the front of their head. As the expression already says, the snail uses its tentacles to feel around in its environment.
 A Roman snail's eye. [RN] |
Those four tentacles can be moved separately and can also be withdrawn, should the snail come in contact with an obstruction. Roman snails do have well developed eyes. Only those are not very good at focusing things. So the tentacles are much more important to improve orientation of the snail in its immediate surroundings.
Basically all snails have got tentacles. The appearance and function of those, however, may be very different in groups not closely related to each other.
In truth, the pulmonate land snails, of which Helix pomatia is one species, are the only snails to possess four tentacles in two pairs.
At the tip of both the larger tentacles there is one eye, appearing as a simple black dot in its natural state. Under a microscope's magnification, though, a relatively highly developed lens eye is revealed. Consequently, the pulmonate land snails scientifically are called Stylommatophora – stalk eye bearers.
 Extending and withdrawing of a tentacle. [RN] |
Another special ability of pulmonate land snails is to withdraw their tentacles separately from each other.
While blood pressure (3) is used to extend the tentacle (see picture on the right, A), the snail has to use muscle power to withdraw the tentacle (B). The retractor, a withdrawing muscle (2) attached to the tentacle's interior, is contracted (4) and the tentacle disappears in the protecting head.
 A Roman snail with its right hand tentacles partly withdrawn. [RN] |
The eye (1) can be followed as a black spot in the tentacle's interior.
While the larger tentacles, as already mentioned, serve as eye stalks, the shorter ones mainly are tactile organs. Besides, both pairs also are equipped with olfactory sense cells.
Special cases are some predatory species of terrestrial snails, such as the rosy wolf snail (Euglandina rosea). This species appears to have six tentacles (Picture below, B). But the appearance is deceptive. The third pair of tentacles actually is the lips situated below the shorter tentacle pair. A Euglandina's lips, carrying olfactory sense cells, are specially evolved to suit the species' predatory behaviour, so to follow the its prey, smaller terrestrial snails, along their slime trail. Not all predatory snail species have such tentacle-like lips, only those tracking their prey along their slime trail, like Euglandina or the South-African Natalina species (Rhytididae family).
 Head and tentacles of Helix (A), Euglandina (B) and Pomatias (C). [RN] |
On the other hand, there are also terrestrial snails that have only two tentacles (C). One of them is Pomatias elegans, the round mouthed snail.
 Round mouthed snail (Pomatias elegans), with shell lid (operculum). Picture: Michael Stemmer. |
There is another difference between Pomatias and Helix: The former's eyes are placed at the tentacles' base. Besides, Pomatias is unable to withdraw its tentacles. Also there is the snout prolonged to an ostentatious proboscis.
Pomatias elegans is indeed a special case: It also carries a shell lid (operculum) at the tail tip of its foot. That almost never occurs among terrestrial snails. Pomatias belongs to the Littorinoidea (order Neotaenioglossa) – its nearest relatives are the sea-living periwinkles (Littorina). There are other terrestrial snail groups carrying an operculum as well, such as point snails (Aciculidae) and operculate wood snails (Cochlostomatidae). Other than the large number of highly adaptive pulmonate land snails, the number of operculate land snails has remained small.
 Ram's horn snail (Planorbarius corneus).[RN] |
There are other snails with a single tentacle pair. Those as well are pulmonate snails, among them the fresh water species of pond snails (Lymnaeidae) and ram's horn snails (Planorbidae). As they have eyes at the base of their tentacles (as does Pomatias), they are called base-eye-snails (Basommatophora).
More closely related to Pomatias are the operculate fresh water snails, such as the mud snail (Viviparus contectus), which, together with apple snails, is placed in the group Ampullarioidea. Mud snails as well, of course, have only two, unretractable, tentacles.
There are also sea-living gastropods, which, like stylommatophoran land snails, have developed eye stalks. But the shells or conches of the Cassidae and Strombidae families need those stalks rather to be able to peer out from under the protective shell without having to expose the vulnerable head.
Systematic
classification of the mentioned groups (partly according to CLECOM).