GENERAL CHARACTERS. 



sisting typically of (a) a loop connecting the two pleurals and 

 provided with two visceral ganglia, and (b) a stomato-gastric 



9f l 



gab 



FIG. 205. Ideal mollusc. After Ray Lankester. 



w., Mouth ; g.c., cerebral ganglia ; c. t edges of mantle skirt ; z.g. t 

 duct of right lobe of digestive gland ; s., pericardial cavity ;_/^, 

 edges of shell-sac ; v., ventricle of heart; u., nephridium ; an., 

 anus ; n., posterior part of the foot ; /., opening of nephridium ; 

 k., genital aperture; g.ab., abdominal ganglion on visceral 

 loop ; g.v., visceral ganglion ; z.l., left lobe of digestive gland ; 

 ^., foot ; g.pe., pedal ganglion; g.pL pleural ganglion. 



loop connecting the cerebrals below the gullet and provided 

 with two buccal ganglia (Fig. 205). Except in Lamelli- 

 brancliS) in which the head region is degenerate^ there is in the 



FIG. 206. Stages in molluscan development. 



/?, Larva of Heteropod (after Gegenbaur) ; sh., shell covering 



visceral hump} z>., velum ;_/!, foot. 

 E, Larva of Atlanta (after Gegenbaur); ., velum; sA., shell; 



yi, foot ; o/>., operculum. 



mouth a chitinous ribbon or radula, usually bearing numerous 

 small teeth, and moved by special muscles, the whole structure 

 being known as the odontophore. There is much unstriped 

 muscle, but the more rapidly contracting muscles have cross 



