Shell ovoid, with short spire, wide aperture and folded columella; inferior pallial lobe thick; visceral commissure still twisted.
Above the oesophagus is a thin commissure which passes laterally into the chief armnerve.
Visceral commissure short, tendency to concentration of all ganglia in dorsal side of oesophagus.
The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in Aplysia the whole nervous system is aggregated in the cephalic region, either dorsally or ventrally.
N 1, Commissure uniting this with ventral ganglion (not shown in fig.).
Owing to the small size of the olfactory lobes the anterior arms of the latter commissure are wanting.
This is shown by the labial commissure and pedal cords of the nervous system, by the opening of the gonad into the right kidney, and by other points.
The pleural ganglia are also united by a long visceral commissure as in Lamellibranchs, and this commissure bears two ganglia lying close beneath the epidermis in front of the anus.
The four cords are all connected anteriorly with the cerebral commissure which lies above the buccal mass anteriorly.
In some cases each pleural ganglion is connected with the opposite branch of the visceral commissure by anastomosis with the pallial nerve, a condition which is called dialyneury; or there may be a direct connective from the pleural ganglion to the visceral ganglion of the opposite side, which is called zygoneury.