CILIARY MECHANISMS OF LAMELLIBRANCHS 633 
the outer faces are smooth. Each fold is laid forward so as to 
cover the groove lying anterior to it, as well asa part of the next 
fold (fig. 3). The folds are not straight, but bend anteriorly 
‘as they are traced from their ventral ends, where they are much 
larger, toward their narrow dorsal ends. It will be observed 
that these folds do not cover the entire face of the palp. On 
its margins, above and below the folds, are narrow, plain tracts 
(dm and vm), and the folds cease, also, a little distance from the 
lateral oral groove. The course taken by food on the palps, 
whether received from the inner gill, or, in Schizotherus, from 
the vascular fold, is directly forward across the folds, as indi- 
cated by a set of arrows in figure 1. It is carried into the lateral 
oral groove, then downward, and in toward the median plane of 
the body, along the proximal part of the groove, to the mouth. 
Food collected by the outer demibranch, coming down the dis- 
tal groove to the lateral, also takes the same course. In their 
passage across the palp folds, particles are influenced by narrow 
lines of cilia found along the centers of the folds, and indicated 
by small, upwardly directed arrows. These lines are very nar- 
row, and particles carried anteriorly across the folds by the gen- 
~ eral sweep of ciliary action, are usually affected for a brief period 
by these cross tracts. The course taken by a small particle placed 
on the palp surface is indicated by the large feathered arrow. 
It will sometimes pass over several folds without feeling the up- 
ward thrust of the fine cilia lines, and then may be carried for 
some distance on some one line. The utility of these narrow 
cilia tracts in the centers of folds, is without doubt, in keeping 
food material away from the ventral mergin (vm); for should it 
touch this, it would at once be turned backward and thrown off 
the posterior end of the palp on to the mantle, which, in turn, 
would cast it from the body. Yet the palp folds of very few 
lamellibranchs have been observed to possess these upwardly 
directed tracts. They were first seen after many forms had 
been studied, and in a good many cases, a re-examination of palp 
folds was not possible; but they probably are absent from most 
palps, for folds of these organs are usually very narrow, and 
material is carried to the lateral groove with certainty. 
