The function of organs in a Roman snail's organism is controlled by the nervous system. The basic nervous system of molluscs is called tetraneural – it consists of four main nerve tracts that extend from a circumesophageal nerve ring around the gullet (esophagus) and connect it to nerve knots or ganglia in important parts of the body. The ganglia process stimuli from sense cells and instructions to organs and muscles.
A Roman snail's circumesophageal nerve ring.
Source: Kilias (1985).
This primordial state today can be found among simple snails, like for example the rock-dwelling limpets. Among the much more highly developed terrestrial snails, there has been a distinct evolution towards concentration of ganglia near the cerebral ring. 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.
Legend of the illustration:
Bg: Buccal ganglion; No: Eye nerve; Cg: Cerebral ganglion; Pdg: Pedal ganglion; Plg: Pleural ganglion; PVg: Parietal and visceral ganglia. 1: Cerebro-pedal connective; 2: Cerebro-pleural connective.
In addition to the ganglia as such there are two types of nerve connections in a snail's nervous system: Commissures link two different ganglia laterally. Connectives link two different ganglia of the same side longitudinally.
In the basic state of the gastropod nervous system, the ganglia were placed in certain parts of the body, where they had the task of processing information regarding that particular part of the organism. In a Roman snail's cerebral ring, however, ganglia, commissures and connectives are molten to such an extent that the single components are not recognisable without difficulty.
Located further in the snail's head, there are the two buccal ganglia, which constitute a further nervous centre, from which nerves lead to stomach and front parts of the intestine, as well as to the salivary glands. By a pair of commissures they are linked to the respective cerebral ganglion.