Snails' Nervous System

Illustration: Nervous system of a prosobranch snail. Resource: Czihak, Langer, Ziegler: Biology. (1992)

Snails' nervous system probably can be derived from the articulates rope ladder nervous system. Today it is thought that molluscs and articulates at least had common ancestors, one hint of which is the similar nervous system and similar larval generations. Among primitive molluscs (Tryblidia) a clearly metamerous segmentation of the nervous system and other organ systems can be seen.

The snails' central nerve system is most concentrated near the head due to concentration of sensual organs at the snail's head end. Here nerve knots (ganglia) are located (cerebral ganglia). From here two pairs of connectives stretch through the rest of the body. These are on one hand the pedal connectives innerving the foot, and on the other the pleurovisceral connectives innerving pallial cavity and visceral sac.

Among primitive snails the pleurovisceral connectives are stretched. During torsion ( see: Snails' body construction) among prosobranch snails the streptoneury (crossing of nerve paths) takes place.

The visceral sac's nerve knot is called visceral ganglion. In contrary to the mostly paired other ganglia this nerve knot is unpaired. The pallial cavity's nerve knots are called pleural ganglia, the lateral ganglia are called parietal ganglia. Among primitive snails between the nerve knots lateral connections, the commissures, can be seen, (whereas the longitudinal connections generally are called connectives). Among more highly developed snails, such as the pulmonate snails, a concentration of nerve knots in the head area has taken place to such an extent, that the term brain can be applied.

The cerebral ganglia are primarily sensual centres, that compute information from the eyes as well as from the tactile and position sensors (statocysts). Besides coordination the also serve the location memory of a snail. The pedal ganglia mainly are necessary for coordination of locomotion. In contrary, the pleural ganglia innerve primarily the mantle, while the parietal ganglia innerve pallial cavity, gills and skin. The visceral ganglion innerves the pallial organs as well as the inner organs.

Around the buccal mass (a muscular construction around the throat) a special buccal ganglion can be present.