338 



MOLLUSC*. 



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

 mouth a chitinous ribbon or radula, usually bearing numerous 



Fig. 



Ideal mollusc. — After Ray Lankester. 



m., 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 ; v., ventricle of heart ; «., nephridium ; an. 

 anus ; «., 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 ; 

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



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

 being known as the odontophore. A portion of the true body 

 cavity or ccelom usually persists as the pericardium at least 



-sh 



Fig. 149. — Stages in Molluscan development. 



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



visceral hump ; v., velum ; /., foot. 

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

 ' f., foot ; op., operculum. 



(Fig. 148, s.), and communicates with the exterior through the 

 nephridium or nephridia. The vascular system is almost 

 always well developed, but part of the circulation is in most 

 cases through ill-defined spaces or lacuna ; the heart typically 



