VII 



MOLLUSCATHE NERVOUS SYSTEM 



139 



cliiastoneury, i.e. the pleurovisceral connectives do not cross (except in Actu'on) ; 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 

 osphradium, 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). J. Tentacular 

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

 ganglion ; 7, osphradium ; S, supraintestinal ganglion ; 9, auditory vesicle ; 10, plcural ganglion. 



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

 of the Diotocardia. 



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

 Cephalaspidae, and Aplysia as representative of the Anaspidic (Apli/siidcc). 



Fig. 119 gives the nervous system of EuUa 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 Actcvoii 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 



