CILIARY MECHANISMS OF LAMELLIBRANCHS 639 
varying conditions is this: When waters are relatively clear, 
small objects, such as diatoms and minute sand grains, or, in- 
deed, any small, chemically non-irritating particles of any nature, 
are brought to the palp folds from the inner demibranch a few 
at a time, and cross them toward the mouth. On the way, they 
are moved dorsalward a little at a time, by the narrow, exposed 
tracts which cross the first, and thus are kept away from the 
ventral margin. On reaching the lateral groove, they are joined 
by the scattered particles collected by the outer demibranch, 
and, deep in the bottom of this groove, they proceed to the 
mouth. Even here, close to the mouth, if their mass extends out 
of the bottom of the groove, they touch outgoing tracts, as will 
be illustrated in the case of Mytilus and other forms, and are 
pulled out and sent away. Everywhere on the palp, a safe 
journey to the mouth depends on the small volume of the moving 
mass. Ihave witnessed the entire journey from gills to digestive 
tract many times in several species, and the success of the experi- 
ment always depended on the presence of a very small amount 
of material. 
Suppose now that conditions are changed—that the water 
becomes filled with silt particles, or, in an experiment, with 
carmine grains or with a mass of diatoms. The outer demi- 
branch makes its collection as before, and, if this is not too 
heavy to remain in the groove on its margin, passes it to the palp 
folds, of course entangled in mucus. It starts across these, 
-uninfluenced by the upwardly directed tracts, on account of its 
mass-and extent. But it proceeds only a short distance before 
muscular movements are observed in the palp. These are two 
in number in Schizotherus, and their adaptations are beautiful 
in their efficiency. Somewhere under, or in front of the advanc- 
ing load, the anterior edges of one or more folds are drawn back- 
ward and upward, so as to expose the ventrally directed tracts 
which have been concealed in the grooves (as at x in figs. 1 and 3). 
Parts of the common mass of mucus and entangled particles are 
driven with great certainty into the grooves, and the whole is 
then dragged swiftly to the ciliation of the ventral margin, there 
to be carried backward, and thrown into the mantle chamber. 
