CILIARY MECHANISMS OF LAMELLIBRANCHS 647 
teriorly, it is directed close to the mantle edge, in order to pass 
outside of a mantle fold, and into a waste canal (wc), as in 
Schizotherus. A sectional view across the waste canal is shown in 
figure 9, wc. That the edges of the mantle folds meet, as is here 
shown, so as to enclose the waste canal, is not a matter of infer- 
ence, but was observed on a living individual. A siphon mem- 
brane, for throwing heavily laden currents downward, is well 
developed, and without doubt, the waste canal protects unde- 
sirable material from being washed back into the mantle chamber. 
The mantle edges being free in the region of the waste canal, 
and Mactra not being entirely buried in the bottom, the col- 
lected waste is not accumulated at the base of the incurrent 
siphon, but is carried out, partly by a feeble ciliary action, and 
partly by muscular contortions of the mantle, between its edges. 
As Mactra buries only the anterior part of the shell in the 
bottom, the mantle edges, including their folds, are often, if not 
always, opened at a point indicated by the feathered arrow in 
figure 8, to form a supplementary incurrent opening, through 
which water is drawn into the mantle chamber, as proved by 
suspended carmine. What advantage is gained by this second 
passage into the chamber of the mantle, is hard to imagine. 
Spisula (Hemimactra) polynyma, Stumpson 
This form was taken from the Bay of Fundy. 
Gills and palps. As shown in figure 10, the ciliation of gills 
and palps is like that of Schizotherus, except that dorsally 
directed currents were not seen on the palp folds. In their 
normal positions, the posterior palps are nearly covered by the 
inner demibranchs, and the outer demibranchs are removed by a 
Fig. 8 Mactra solidissima; mantle ciliation; m, mantle wall; we, waste canal. 
Fig. 9 Mactra solidissima; section across mantle edges; sh, shell. 
Fig. 10 Spisula polynyma; ap, anterior palp; b, bay of mantle; dg, distal 
oral groove; f, foot; ig, inner demibranch of gill; og, outer demibranch of gill; 
s, siphon; w, wall of waste canal; we, waste canal. 
Fig. 11 Spisula planulata; letters as in figure 10. 
Fig. 12 Saxidomus gigantius; mantle ciliation; b, bay of mantle; m, mantle 
wall; sl, shell ligament; sm, siphon membrane. 
