YII 



MOLL use A— THE NERVOUS SYSTEM 



139 



chiastoneuiy, i.e. the jileurovisceral connectives do not cross (except in Actami) ; and 

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

 of the pharynx. 



(«) Tectibranchia. — As a rule only the right parietal ganglion is found (in Acta:on 

 the left is also present). A nerve vises 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. lis.— Nervous System of Cyclostoma elegans (afte-r Lacaze-Duthiers). i. Tentacular 

 nerve; 2, eve; 3, ct-rebral sranglion ; 4, peilal sanglion : 5, infraiiitestiual trantjlion ; 6, vi.sceral 

 ganglion ; 7, osphi'adiuin ; S, supraiutestinal ganglion ; 9, auditory vesicle ; 10, jileural 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 

 Cephalaspidce, and Aplysia as representative of the Anaspidcc {Apli/siidce). 



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 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 



