Mollusca. 



rasping, or prehensile organs. The members of this division have Mollusca. 

 a bivalved shell, and are either fixed, or sessile, or endowed with GALLEBIES 

 very feeble powers of locomotion. 



Dorsal margin. 



Anterior side. 



Posterior side. 



Ventral margin. 



Dorsal margin. 

 I. 



Posterior side. 



Anterior side. 



Ventral margin or base. 



Fig. 12. — The common fresh -water mussel, Unio pictorum^ Linn. (A) 

 exterior view of left valve ; (B) interior view of left valve, showing 

 animal ; the right valve is removed. «, umbo ; I, ligament ; o, the 

 mouth ; p,p, pedal muscles ; a, a, adductor muscles ; /, foot ; b, branchial 

 orifice ; v, anal opening ; t, palpi. The arrows in both figures indicate 

 the incurrent and excurrent openings. 



Fio. 13. — View of underside of pond-snail, Limncea stafftialis, Linn., 

 aa seen crawling on the side -glass of an aquarium. /, the foot. 



