344 



MOLLUSC A. 



ribbon or radula, usually bearing numerous small teeth, and 

 moved by special muscles, the whole structure being known as 

 the odontophore. A portion of the true body cavity or ccelome 



9f 



FIG. no. Ideal Mollusc. (After RAY LANKESTER. ) 



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

 duct of right digestive gland ; s., pericardial cavity ; f., edges of 



usually persists as the pericardium at least (Fig. no, s.\ and 

 communicates with the exterior through the nephridium or 

 nephridia. The vascular system is almost always well 



of 



FIG. in. Stages in Molluscan development. 



Z>., Larva of Heteropod (after Gegenbaur); sk., shell covering 

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



., Larva of Atlanta (after Gegenbaur); v., velum; s/i., shell; 

 f., foot ; op., operculum. 



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

 ill defined spaces or lacunce. Respiratory organs are most 

 typically represented by a pair of vascular processes of the 



