MUSSELS AND SNAILS. 
159 
the water the minute animals and plants which nourish 
the clam. These bits of food are collected along the 
ridges on the gills and thence passed to their ventral 
edges, on each of which there is a groove leading to the 
mouth. Along this groove the food passes, whipped 
on by the cilia, until it enters the mouth, which is 
Fig. 124. —Fresh-water Mussel with One Valve Removed. p, peri¬ 
cardium; p.a., posterior adductor muscle; a.a., interior adductor 
muscle; a , anus; e.s ., exhalent siphon; i.s., inhalent siphon; l.m., 
cut edge of mantle; o.g., outer gill; v.g ., inner gill; /, foot; l.p., 
labial palps. 
situated between the labial palps at the anterior end of 
the body. 
To understand what becomes of the water that brings 
in the food we must remember that each gill is a fold 
of membrane pierced with tiny holes. In order to pass 
from the branchial chamber to the cloacal chamber the 
water must pass through these holes. A cross-section 
of a gill has a V shape, and the water goes through the 
sides of the V into the opening above and thence passes 
out of the cloacal chamber by the upper siphon. 
