VII 



MOLLUSCA—THE NERVOUS SYSTEM 



139 



uhiastoiieury, i.e. the pleurovisceral connectives do not cross (except in Actceon) ; 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 Actmon 

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

 osphradium, and the mantle, and foi'ms a branchial ganglion at the base of the gill. 

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



Fig. us.— Nervous System of Oyclostoma elegans (after Lacaze-Duthiers). 1, Tentacular 

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

 ganglion ; 7, osphradium ; S, supraintestinal ganglion ; 9, auditory vesicle ; 10, pleiu-al ganglion. 



commis.sure below the pharynx, and may be compared with the labial commissure 

 of the Diotocardia. 



As types of the Tectibranchia we may take Sulla as reja-esentative of the 

 Cephalaspidce, and Aplysia as representative of the Anaspidw (Aplijsiidce). 



Fig. 119 gives the nervous system oi 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 cerebropleural connectives becoming correspondingly 

 shortened. (In Actaion 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 Cephalaspida:, moreover, no distinct right parietal ganglion exists. It 



