An investigation of the cerebral nervous system was made in one caenogastropod and 59 heterobranch gastropods and gave the following results. Each order of the class Gastropoda shows characteristic features in the shape of its cerebral nerves. Homologies of the cerebral nerves in different gastropod taxa are recognized on the basis of their position and function. Consequences for the systematics of Gastropoda are a separation of Architectonicidae and Pyramidellidae, formerly united in the clade Allogastropoda (Hasz-prunar, 1985a); a derivation of the Archaeopulmonata from below the level of basal ‘cephalaspids’.
The tentacles of Caenogastropoda, Architectonicidae, Omalogyridae, Rissoellidae, Pyramidellidae and Pulmonata as well as Thecosomata and veligers are innervated by the Nervus tentacularis which can be homologized with the Nervus clypei capitis of the ‘Cephalaspida’, Aplysiomorpha and Sacoglossa where it innervates the headshield or head. Homologous nerves are lacking in Umbraculomorpha, Pleurobranchomorpha, Anthobranchia, Nudibranchia, and Rhodopemorpha.
The cerebral nerves in Architectonicidae and Archaeopulmonata show distinct similarities with those in Caenogastropoda.
The Nervus rhinophoralis, which in Opisthobranchia innervates the posterior portion of Hancock's organ, is also found in Pyramidellidae, but is lacking in the Archaeopulmonata.
The position of the optic nerve is identical in Caenogastropoda, Architectonicidae, Omalogyridae and Archaeopulmonata on the one hand, in Rissoellidae, Pyramidellidae and ‘Cephalaspida’ on the other hand, and therefore is considered to have systematic importance. In contrast to Caenogastropoda, Architectonicidae, Omalogyridae and Rissoellidae, both the Pyramidellidae and Euthyncura possess a Nervus lateralis and giant cerebral nerve cells.
The cerebral nerves of the Thecosomata reflect a larval condition.
Within the investigated groups, small species show a reduction in the number of their cerebral nerves, whilst small opisthobranchs have an increased number of accessory ganglia.