CILIARY MECHANISMS OF LAMELLIBRANCHS 689 
point just posterior to the palps (below the reference letters dm) 
where it collects, if not at once transferred to the apposed palp 
faces. Collections are also brought downward to this point, 
along the demibranch edge lying parallel with the distal oral 
groove. It is probable that in all bivalves, the palps may be 
withdrawn from contact with this demibranch edge, so that 
accumulations on the margin become great enough to fall of 
their weight to the mantle or visceral mass walls. 
Palps. Unio is one of those forms in which the dorsal edges 
of the lateral extensions of the palps are united so as to bring 
the sets of folds close together, leaving but a very narrow tract 
(the lateral oral groove) between them (fig. 64, lg). Where the 
distal joins the lateral part of the oral groove, the dorsal margins 
make a short trough (to the left of reference letters ap). The 
ventral palp margins (vm) in the Unios and Anodons are narrow, 
as are the folds, and observation of their ciliation is difficult. 
The wide dorsal margin of the posterior palp receives much 
material collected by the visceral mass in its vicinity; that of the 
anterior, in the same way, receiving mantle collections on its 
inner, and also on its outer face from contact with the mantle. 
These points are not well shown in the figure. The outer faces 
of both palps are ciliated, and bear material to the free dorsal 
margins, where it comes over on to the inner faces. None of it 
is moved to the folds, but all is carried to the free tips and thrown 
off. 
The general ciliation on palp folds is nearly straight across 
them, forward. The details of the ciliation on the folds are 
Fig. 63 Unio complanatus; es, excurrent opening of siphon; f, foot; 7g, inner 
demibranch of gill; 7s, incurrent opening of siphon; 0g, outer demibranch of gill; 
p, palps. 
Fig. 64 Unio complanatus; ciliation of palp region; aa, anterior adductor; 
ap, anterior palp; dg, distal oral groove; dm, dorsal palp membrane; ig, inner 
demibranch of gill; lg, lateral oral groove; m, mantle, og, demibranch of gill; pp, 
posterior palp; vm, ventral palp margin; vms, visceral mass. 
Fig. 65 Unio complanatus; detail of ciliation of palp folds; fd, fd, fd, palp 
folds; vm, ventral palp margin; x, direction of ciliation across folds; y, separation 
of folds to expose deep cilia tract; z, course of particles over deep tract. 
Fig. 66 Anodon sp.; mantle ciliation; es, excurrent opening of siphon; ‘s, 
incurrent opening of siphon; m, mantle wall; p, palp. 
