NERVOUS SYSTEM — GASTROPODA. 
213 
In the Mollusca there are generally four chief nerve masses or 
ganglia, which may be broadly classified as Supra-oesophageal and 
Sub-oesophageal, according to their position above or beneath the 
alimentary canal. The supra-oesophageal neiwe mass is composed 
solely of the Cerebral or Sensory ganglia, while the compound sub- 
oesophageal group is formed by the Pedal or Motor ganglia, and the 
Visceral and the Buccal or Stomato-gastric centres, which to a certain 
extent are comparable to a sympathetic system. The cerebral and 
pedal ganglia are essentially nerve centres for the ectodermic organs, 
the buccal ganglia for those of the endoderm and the combined 
visceral centre for those of mesodermic origin. All these centres 
are each more or less distinctly paired in correlation with the bilateral 
arrangement of many of the organs of the body, but they are all 
Fig. 421. — Diagrammatic obliquely dorsal view of the nervous system of a Gastropod, showing 
the four nerve centres and their commissures enclosed separately within dotted lines. 
b.g. paired buccal ganglia (the curved line and arrow indicate the change in position they undergo 
in some species by the retraction of the buccal bulb) ; c.g. paired cerebral ganglia ; p.g, paired pedal 
ganglia ; the compound visceral centre is also enclosed by a dotted line and is formed by the 
pleural ganglia, pa.^. pallial ganglia, and ab.g, abdominal ganglion. 
liable to vary in size and importance and in the amount or mode 
of fusion with each other or with neighbouring ganglia in accordance 
with the specialization the animal or its organs have undergone. 
The Gastropoda, in their nervous system, exhibit throughout our 
British species the four normal paired nerve centres, which form three 
more or less confluent nerve loops encircling the alimentary canal, 
all of which arise from the supra-oesophageal ganglia, and, according 
as they do or do not exhibit a twist or crossing of the posterior 
nerve cords, have been distinguished as Streptoneura and Euthyneura 
respectively. 
The Streptoneura or Chiastoneura constitute a group practically 
co-exteusive with the older gvoups Prosobranchiata and Operculata, 
and is composed of the most primitive and archaic of our species, the 
nervous system being still characterized by the innervation of the male 
organ of reproduction from the pedal ganglia, by the widely separated 
visceral centres, and more especially by the remarkable crossing of 
the pleuro-abdominal commissures arising from the semi-rotation of 
the visceral sac. 
