Nervous System

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.

• Cerebral ganglia: The two cerebral ganglia are nearest to a snail's “brain”. Each cerebral ganglion consists of three different parts, comparable to a forebrain, a middlebrain and an afterbrain. From the forebrain, nerves reach to the tentacles, the optical nerve and the olfactory nerve, as well as to the penis (!), from the afterbrain there are nerves to lips (labiae) and equilibrium organ (statocysts). The middlebrain finally is the computing centre of all of a snail's cerebral nerves. By connectives it is also linked to the other ganglia, by a commissure it is linked to the second cerebral ganglion.
   
• Pedal ganglia: The pedal ganglia make the lower part of the circumesophageal nerve ring. Nerves from here mainly connect to muscles in the foot.
   
• Visceral ganglia: The rear and upper part of the circumesophageal ring consists of the large, single ganglion, which, as its name says, processes information from the internal organs and organ systems. In truth, though, this “visceral ganglion” is made from five different ganglia, in simply built limpets still located in different parts of the body, but in a Roman snail's organism molten to form one hyper-ganglion:
  • Pleural ganglia: There are two pleural ganglia making the most external part of the visceral ganglion. Nerve paths from here among others lead to the gullet retractor muscle and the main retractor muscle (columellar muscle).
  • Parietal ganglia: More inside there are the two parietal ganglia, which are the starting point of the mantle nerves. They are hardly recognisable as separate ganglia as they are very much molten to the visceral ganglion proper.
  • Visceral ganglion s. str.: In the centre, the visceral ganglion is the actual ganglion for the internal organs. From here there are nerve pathways to organ systems of digestion, genital apparatus, as well as heart and kidney.

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.