Neogastropod Nervous System
(typical of the superfamily, Muricoidea)

(Left) Anatomic drawings of the nervous system in two different gastropod orders, taken from Lankester, 1906 (modified). In the upper figure, a simpler, diffuse nervous system is shown. This is characteristic of primitive grazers like the limpet. In the lower figure, a more centralized nervous system is shown, which is typical of many advanced, carnivorous snails. Overall, the nervous system, unlike other organ systems, retains the bilateral symmetry thought to be characteristic of ancestral, primitive molluscs.

Both gastropod clades basically show the following: paired cerebral ganglia; paired pleural ganglia, innervating the lining of the mantle cavity; paired pedal ganglia, innervating the foot; and a more variable set of visceral ganglia. All have one or more "commissures;" i.e., nerve trunks connecting the right and left sides of the "brain."

Patelloidea species, in the Clade: Patellogastropda, generally, have two widely separated major ganglia, the pleural and the pedal, which are connected by rather long nerve trunks (e.g., cerebro-pleural connectives in upper figure). Ganglion separation of this kind is also characteristic of bivalves, which also have a relatively simple nervous system.

Species in the more evolutionarily advanced Clade: Neogastropoda, by comparison, show a migration of the major ganglia to the head region, where they have become densely consolidated and interconnected without long trunks. Smaller ganglia, like the buccal ganglia, have now become consolidated in the cerebro-pleural complex. The overall consequence of this consolidation is to enable measureable processing intelligence, as discussed below.

Sense organs generally include: 1) a statocyst (red, in the figures) for positioning sense; 2) eyes (see for example, Strombus, Pecten, Chiton eyes), or if not differentiated eyes (yellow, in the figure), at least photosensitive nerve endings connected via the optic nerves; 3) tactile or touch receptors located at various nerve endings; and 4) a chemosensitive "taste" organ, the osphradium (blue, in the upper figure). While the osphradium is not shown in the lower figure, it would be connected via the short nerve trunk extending  below the visceral ganglion.

Environmental perception in gastropods seems to be based primarily on chemoreception through the taste organ (Kohn, 1961; Audesirk & Audiserk, 1985).

For many years, visualization has been considered to amount to little more than a primitive response to light or dark. But in the higher orders of opsithobranchs and pulmonates, the molluscs show orientation with respect to grass stalks or orientation with respect to celestial cues (Hamilton, 1977; Hamilton & Russell, 1982). It seems that the "brain" has formed an internal representation of important environmental features.