GENERAL CHA RA C TERS. 



365 



sisting typically vf(a.) a loop connecting the two pleurals and 

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



c Z 9 



an 



37 



yr ' */ .</' 



I'lG. 181. Ideal mollusc. After Ray Lankester. 



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

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

 edges of shell-sac ; ?'., ventricle of heart ; ?/., nephridium ; a>i., 

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

 /., genital aperture; fr.afr., abdominal ganglion on visceral 

 loop ; ^-.r 1 ., visceral ganglion ; z.L. left lobe of digestive gland ; 

 p., foot ; g.pe., pedal ganglion ; g.pl., pleural ganglion. 



connecting the cerebrals below the gullet and provided with 

 two buccal ganglia (Figs. 181, 188). Except in Lamelli- 

 branchs, in which the head region is degenerate, there is in the 





og-s 



of 



FIG. 182. Stages in Molluscan development. 



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

 visceral hump ; z 1 ., velum ;f., foot. 



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

 f, t foot ; op., 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. The muscles are predom- 

 inantly non-striped. A portion of the true body cavity or 



