CILIARY MECHANISMS OF LAMELLIBRANCHS 635 
ment. As a-matter of fact, except when waters are very heavily 
laden with sediment, lamellibranchs keep their shells open, and 
allow large quantities of material to be brought into the mantle 
chamber. ‘This is deposited on the surfaces of all organs exposed, 
but especially on the gills, or food collecting organs. Except in 
the cases of Yoldia, Monia, and Pecten, among the forms exam- 
ined, gill collections can only be moved on to the palps, though 
experiment would indicate that exceptionally large quantities 
might fall off from their margins, the masses being too great to 
be retained by their grooves. 
The surfaces of all organs exposed in the mantle chamber pro- 
duce mucus which is poured out in response to the touch of for- 
eign particles. This is true of mantle and visceral mass, as well 
as of gill and palp, the adaptation everywhere being the same— _ 
the entanglement of solid substances. 
Mantle ciliation. Considering the organs concerned in the 
removal of material from the body, the mantle folds, one of which 
is illustrated in figure 2, bound the great cavity, and much of the 
material brought in by the waters adheres to them. At once, 
mucus and entangled particles begin to move downward, and 
toward a point near the center of the ventral mantle edge. Fig- 
ures 1 and 2 attempt to show that the mantle edges are fused 
from a point, vz, posteriorly. At the sides of these fused ven- 
tral margins arise erectile walls, the left one, w, being shown in 
the figures. These walls are united at the base of the siphon, 
slightly above the reference letter b. They may be elevated 
and bent toward each other until they almost or quite meet. 
(Their actual union has been observed in Mactra.) They thus 
enclose a chamber or canal which will be called the waste canal 
(we). Similar structures occur in other forms to be described. 
The collections of the mantle folds enter the open anterior end 
of the canal, as indicated in figure 2, and are carried swiftly 
backward to be accumulated in a mass at b. This mass may 
become nearly half an inch in diameter in Schizotherus. 
In figure 2, and other figures showing the direction of mantle 
currents, it will be noticed that immediately in front of the base 
of the incurrent siphon through which the stream enters, the 
