400 



STRUCTURE OF LAMELLIBRANCHIATA. 



function of respiration, is effected. In many cases (when the 

 two sides of the mantle are entirely united, except at the respi- 

 ratory openings), the existence of these currents is indispensable 



Foot 



Intestine ! 



Stomach 



Gills 



Mantle 



Anus 



Respiratory Tubes 

 FIG. 686. ANATOMY OF AN ACEPHALOUS MOLLUSK. 



not only for respiration, but also for the nourishment of the ani- 

 mal, as the minute particles of organic matter are separated by 

 the gills, by a sort of filtering process, from the water in which 

 they are floating. According to Messrs. Alder and Hancock, 

 the structure which enables the gills to perform their multifarious 



