CILIARY MECHANISMS OF LAMELLIBRANCHS 673 
brought into the mantle chamber a few at a time in relatively 
clear water. 
There is an entirely different disposition of particles when they 
are numerous, which is illustrated in figure 43. The stipples at 
the level a represent many particles, some having lodged in the 
grooves, others on the folds. They are numerous enough on the 
folds and in the grooves to be imbedded in a common mass of 
mucus, and instantly a strife begins between opposing streams 
for the control of the mass, as shown at 6. The mucus being 
sufficient to hold together, it is evident that the downward thrust 
on the crests of the plicae tends to lift that part of the common 
mass resting in the grooves, and finally raises it until it loses 
contact with the groove cilia, which lash upward. Being drawn 
entirely out of the grooves, the whole is pushed rapidly downward 
on the crests of the folds to the free margin of the demibranch. 
Here there is a tendency to hold it in the groove leading to the 
palps, but the mass is usually too large, and falls off into the 
mantle chamber, where it will be picked up and carried out of the 
body. Between the two demibranchs, a tract leads forward to 
the palps (fig. 44, g). 
This disposition of gill collections must inevitably occur when 
the entering water bears silt, or any other suspended particles, 
in a certain definite quantity. Silt, very fine sand, carmine grains, 
and even diatoms, have been used in the experiments, and in all 
cases, when their volume becomes great enough, the gill casts 
them off as described. With a little experience, while the intro- 
duced. mass is settling in the water toward the gill face, it may 
quite accurately be predicted whether it will take the paths to 
the palps, or be gathered up and east off the free margin into the 
mantle chamber. It was proved here, as well as in all other 
cases, that there is no recognition and selection of particles suit- 
able for food in matter brought into the mantle chamber, but 
that the volume of material alone, of whatever nature, deter- 
mined whether it would reach the mouth or not. 
Muscular movements. Because of the length of this account, 
only brief reference has been made to muscular movements, 
common to all palps and gills, which aid the cilia in ridding 
