Nervous system
The typical nervous system of gastropods are affected by torsion to give rise to
streptoneury, which results in the crossing of
cerebrovisceral commissures in a figure of eight pattern. This disrupts the original bilateral symmetry of the nervous system which consists of paired cerebral, pedal, buccal, pleural ganglia along with several others which are associated to the visceral mass (Bulloch & Ridgeway, 1995).
Figure 5. Diagram of the state of streptoneury
found in prosobranch gastropods,
Haliotis and
Littorina. Adapted from Kandel, 1979; Croll, 1987.
Cephalization of the nervous system have also occurred in less primitive gastropods where shortening of distances between the different central ganglia are the result of shortening of connectives and commissures. This could be the precursor of ganglionic fusion, leading to a central nervous system (Bulloch & Ridgway, 1995).