CILIARY MECHANISMS OF LAMELLIBRANCHS 691 
above the margin, not touching and interfering with its current. 
The folds lie over anteriorly—toward the right of the figure— 
each covering a part of the base of the one in front of it, in the 
usual manner. The line of arrows x represents the general 
course taken by particles across the folds toward the mouth. 
On each fold, close to the edge of the fold behind it (as just above 
the letter y) there are very narrow tracts, the cilia on which 
direct currents dorsalward toward the lateral oral groove. These 
have some influence on material crossing the folds, tending to 
keep it away from the ventral margin, on which it would be ear- 
ried away from the mouth. When much material appears on 
the folds, they lift here and there (y), so that it is caught quickly 
and certainly by cilia deeper in the grooves, and, as represented 
by the dotted arrows, is carried to the outgoing stream on the 
ventral margin, precisely as in Schizothreus, Cardium, and others. 
Mantle. Figure 66 represents the ciliation of an Anodon, which 
is in most respects similar to that of Unio. In both, material is 
carried out over the margin below the lower siphonal opening 
(7s), as is usual in forms not completely buried. 
Yoldia limatula 
If this much discussed genus is properly placed among the 
most archaic of living lamellibranchs, this representative of it 
certainly possesses the most extraordinarily complex set of 
ciliary mechanisms observed in the group. It lives in soft mud, 
and, as shown by the stomach contents, allows relatively large 
quantities of indigestible material to pass into the digestive 
tract, though the relative volume of diatoms found, is very much 
greater than in Macoma. Yoldia does not, by any means, feed 
simply on what it is able to digest out of the mud of the bottom 
in which it lives, for diatom shells would not in that case form 
so large a part of the stomach contents; and several very elabor- 
ate and effective mechanisms exist, the function of which is to 
clear the organs exposed in the mantle chamber of objectionable 
material brought by the incurrent stream. 
In order to understand the operation of these mechanisms, it 
is necessary to have a clear notion of the relative positions of 
JOURNAL OF MORPHOLOGY, VOL. 26, No. 4. 
