CILIARY MECHANISMS OF LAMELLIBRANCHS 699 
ward, and leaving the mantle chamber at the point, just below 
the siphon, where the palp tentacles are extruded. 
SUMMARY 
Numerous anatomical features, not previously noticed, are 
shown in the figures. 
Unique organs in the group of the lamellibranchs, are the 
immerse vascular fold arising from the visceral mass of Schizo- 
therus (fig. 1), and the collecting membrane of Barnea costata 
and B. pacifica, also arising from the visceral mass (figs. 57 and 
60). The former is connected with the palps, the latter confined 
to the posterior wall of the visceral mass, and they are probably 
not homologous structures. The direction of cilia currents is 
quite different in the two cases. The organ in Barnea is de- 
veloped to aid and perfect the outgoing cilia mechanism, and 
apparently ean have no other function. The fold of Schizotherus 
is an immense blood reservoir, and its cilia to some degree aid 
the outgoing system. 
The direction of the beat of cilia is never changed. 
Cilia streams on the surfaces of organs exposed to the water 
are divided into two systems, namely, those leading to the mouth, 
and those bearing material outside the body. The operation of 
each, in Schizotherus, is described in detail. 
The numerous currents of all gills belong to the ingoing system, 
except, among the forms examined, Yoldia, Monia and Pecten, 
in which the gills possess certain outgoing tracts which are en- 
tirely different each from the others. Each of these ciliations is 
complex and extremely efficient. 
The palps, which have attained their present size, form, and 
position on account of the function of their cilia, are shown to 
exercise general control over the two ciliary systems, determin- 
ing whether or not collected material shall enter the digestive 
tract. The cilia tracts of the palps are shown to have greater 
complexity and to be more important than those of other organs, 
and this account deals most fully with their positions and actiy- 
ities. Practically nothing has heretofore been published con- 
