VII 



MOLLUSCA 



277 



or supra-oesophageal dorsal in position (c), the pedal ventral (j)), and the 

 pleural somewhat lateral {pi). Uniting these ganglia are commissures, 

 cerebral, pedal, cerebro-pedal, cerebro-pleural, and pleuro-pedal. Further 

 there is a long pleural commissure— the visceral loop (?;./)— on the course 

 of which there are typically three smaller ganglia — a visceral and two 

 parietal. An important point' to notice is that each of the latter is 

 linked on to the ctenidium of its side by a small nerve passing to it 

 from the osphradium — a special sense organ situated at the base of the 

 ctenidium. Connected with all the gangha that have been mentioned 

 are peripheral nerves uniting them with the regions of the body which 

 they innervate. Thus the cerebral ganglia are linked up with the organs 

 of the head, the pedal with the foot, and the pleural with the mantle. 



Fig. 114. 



The central nervous system of Mollusca, A, Gasteropod (enthyneurous) ; 5, Gasteropod (strepto- 

 neurous) ; C, Pelecypod. c, Cerebral (supra-oesophageal) ganglion ; /), pedal ganglion ; pi, pleiural 

 ganglion ; v.l, visceral loop. 



A typical nervous system such as the above is well seen in one of the 

 ordinary fresh- water snails (Limnaea) — air-breathing Pulmonates which 

 have taken to an aquatic existence — except that in them the visceral 

 loop is quite short so that the visceral and parietal ganglia lie close 

 to the other ganglia. In the majority of gasteropods the visceral 

 loop is long and the nerve joining the parietal ganglion to its osphradium 

 being relatively short the parietal ganglion attached to the originally 

 right ctenidium has had to accompany this ctenidium in its shifting 

 during the rotation of the visceral hump. The result is that the origin- 

 ally right parietal gangUon is now on the left, and that the originally 

 right half of the visceral loop has been carried over the left half in 

 X-shaped fashion (Fig. 114, B). This is the twisted or streptoneurous 

 condition of the visceral loop which is contrasted with the primitive 

 untwisted or euthyneurous condition, Streptoneury occurs as stated in 



