VII 



MOLLUSCATHE NERVOUS SYSTEM 



139 



chiastoneury, i.e. the pleurovisceral connectives do not cross (except in Actcvon) ; and 

 (2) by a marked tendency to concentration of the ganglia around the posterior end 

 of the pharynx. 



(a) Tectibranchia. As a rule only the right parietal ganglion is found (in Actceon 

 the left is also present). A nerve rises from it which innervates the ctenidium, the 

 osphraclium, and the mantle, and forms a branchial ganglion at the base of the gill. 

 A delicate lower cerebral commissure is often found, which runs along the pedal 



FIG. 118. Nervous System of Cyclostoma elegans (after Lacaze-Duthiers). l, Tentacular 

 nerve ; 2, eye ; 3, cerebral ganglion ; 4, pedal ganglion ; 5, infraintestinal ganglion ; 6, visceral 

 ganglion ; 7, osphradium ; 8, supraintestinal ganglion ; 9, auditory vesicle ; 10, pleural ganglion. 



commissure below the pharynx, and may be compared with the labial commissure 

 of the Diotocardia. 



As types of the Tectibranchia we may take Bidlct as representative of the 

 Cephalaspidce, and Aplysia as representative of the Anaspidcc (Aplysiidce}. 



Fig. 119 gives the nervous system of Bulla hydatis ; only three points concern- 

 ing it need be mentioned : (1) The pleural ganglia have shifted till they lie close 

 to the cerebral ganglia, the cerebro pleural connectives becoming correspondingly 

 shortened. (In Actceon these ganglia have even fused, and are no longer to 

 be distinguished externally.) (2) There are three visceral ganglia. (3) The 

 commissures are comparatively long. (4) The parapodia are innervated from 

 the pedal ganglia. 



In many Cephalaspidce, moreover, no distinct right parietal ganglion exists. It 



