344 



MOLLUSC A. 



ribbon or radula, usually bearing numerous stnall teeth, and 

 moved by special muscles, the whole structure being known as 

 the odontophore. A portioti of the true body cavity or ceelome 



Fig. iio.— Ideal Mollusc. (After Ray L.^nkester.) 



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

 duct of right digestive gland ; 5., pericardial cavity; /., edges of 

 shell sac; z/., ventricle of heart ; z^., nephridium ; an., anus; «., 

 posterior part of the foot ; /., opening of nephridium ; h., genital 

 aperture ; g.ah., abdominal ganglion on visceral loop ; g-i'-, visceral 

 ganglion ; s./., left digestive gland ; /., foot ; g.pc, pedal ganglion ; 

 g-pL, pleural ganglion. 



usually persists as the pe?-icardiuin at least (Fig. no, s.), and 

 communicates ivith the exterior through the nephridium or 

 nephridia. The vascular system is almost always well 



Fig. III. — Stages in MoUuscan development. 



Z>., Larva of Heteropod (after Gegenbaur) ; sh., s 

 visceral hump ; z'., velum \ /., foot. 



E., Larva of Atlanta (after Gegenbaur) ; c, \-elun 

 /., foot ; op., operculum. 



covermg 



■t/i., shell; 



developed, but part of the circulation is in most cases through 

 ill defined spaces or lacuiue. Respiratory organs are most 

 typically represented by a pair of vascular processes of the 



