CILIARY MECHANISMS OF LAMELLIBRANCHS 645 
Mactra solidissima 
Gills. As in Venus, currents are downward to the free edges 
of the demibranchs of all lamellae, and forward on the margins 
to the palps. There is also a forwardly directed current between 
demibranchs at their bases, but none at the bases of the outer 
lamella of the outer, or inner lamella of the inner demibranchs. 
Palps. These organs are long and narrow, and writhe and 
twist at the touch of small quantities of material. Dorsal and 
ventral margins are relatively wide. In figure 7, the gills are 
removed, and the posterior palp of the right side is shown. The 
course toward the mouth across the folds is indicated, as is the 
fact that the ventral margin carries particles posteriorly to the 
palp tip, where they are thrown off into the mantle chamber. 
The figure indicates, also, that material collected on the back 
face of the palp—that which is applied to the visceral mass—is 
carried over on to the dorsal margin of the inner face. Here its 
general course is backward to the tip, as on the ventral margin. 
Material carried close to the folds is usually caught up and sent 
forward across them. On the folded area, as in Schizotherus, 
there is one set of currents directed ventrally, and another dor- 
sally. These were seen, but their positions not clearly deter- 
mined, before the beautifully clear demonstration on the palp 
of Schizotherus was found. Since then, there has been no op- 
portunity for comparison. It appeared that the ventrally 
directed currents were on the folds, those toward the dorsal 
margins, in the grooves, a condition the reverse of that in 
Schizotherus, but this may be an error. 
Visceral mass. High on the side of the visceral mass, the 
trend of a general ciliation is forward (fig. 7). Part of the col- 
lection here is sent on to the dorsal margin of the palp, which 
organ is extensively united to the body wall. Below this, the 
visceral mass collections are carried to its posterior wall, and 
cast off to fall to the mantle. 
Mantle. There is a single line, or tract, running through the 
general ciliation of the mantle wall (fig. 8, m). Forward, its 
position is farther from the mantle edge than in Venus. Pos- 
