FOOD AND FEEDING IN FRESH-WATER MUSSELS. 
459 
(as described on p. 444), thus widening the furrows, more particles would fall into 
them. These would drop upon the large cilia just below the anterior groove, upon 
those below the posterior groove on the next ridge ahead, or even to the bottom 
of the furrow, where there are also cilia. The cilia in the furrow between the two 
notches beat downward and carry particles to the lower edge of the palp, where 
they pass back to the lower posterior corner and off. To summarize, the cilia 
shown in Figures 15 and 16 beat forward in the region a b c and downward in region 
c d a. 
LONGITUDINAL GROOVE ON FACES OF PALPS. 
Cross sections were made of both juvenile and adult palps (figs. 17, 18, and 20), 
these paralleling, of course, the vertical ridges. As will be noted from the figures, 
there is a well-marked ciliated groove running longitudinally along the inner face 
(i. e., the side next to the outer palp) of the inner palp immediately below the line 
of union of the two palps. The groove is well defined in both the juvenile and 
the adult. There are indications of a smaller, less marked groove at a similar 
region on the outer palp, but it is much less conspicuous. Recalling the direct 
observations on ingestion, in numerous cases material was seen to pass forward' 
between the palps quite near the dorsal margins. Apparently the material 
observed near the dorsal edges was being carried forward in the ciliated grooves 
described. In the experiments in which the mussels were feeding in a heavy 
suspension of debris all the material that was ingested passed between the palps 
in this upper region. The grooves were so widened and filled that the lower part 
of the palps was occupied mostly by masses of material streaming downward and 
off. It would seem that the longitudinal grooves, or at least the large one on 
the inner face of the inner palp, act as an accessory mechanism for carrying 
material forward. Especially in the case of a heavy suspension of particles in the 
water, the action of the groove or grooves enables the mussel to obtain some 
material, although most of the palp is entirely engaged in removing the over- 
abundance of debris. 
Figure 19 shows a portion of the outer face of the palp of an adult mussel. It 
will be seen that it is ciliated. Allen (1914) and Kellogg (1915) agree that the cilia 
on the outer faces of the palps beat away from the mouth and remove material 
from the palps. No observations were made on this matter by the present authors. 
CONCLUSIONS. 
The evidence of direct observation of the process of ingestion does not in the 
least favor the view that the mussel exercises any selection of ingested particles by 
means of the gills or palps. The palps act quantitatively, not qualitatively. 
The palps appear to be a mechanism for reducing the quantity of material to an 
amount that can be handled by the mouth. 
The stretching or lengthening of the palps, as just stated, allows more mate- 
rial to be swept off, so the quantity brought up to the mouth can be regulated to 
some extent in this manner. The experiments in the ingestion of carmine showed 
that at times the mussels closed the valves, usually abruptly, and often kept them 
so as long as carmine grains were present in the water about the siphons or edges 
