CILIARY MECHANISMS OF LAMELLIBRANCHS 649 
is no mantle waste canal. It may be that the diaphragm con- 
trols the amount of water entering the mantle chamber. 
Mya arenaria 
This form is found on the Atlantic from Virginia north to 
Labrador, and on the Pacific, where it has been introduced with 
Atlantic seed oysters, in San Francisco Bay, Puget Sound, and 
northward. 
Gills. As in the majority of lamellibranchs, cilia streams are 
downward to the margins on all lamellae. There is an anteriorly 
directed stream between the bases of demibranchs on each side 
of the body, but none at the bases of the outer lamella of the 
outer, or inner lamella of the inner demibranchs. 
Palps. The general ciliation (one without definite lines or 
tracts) covering the outer faces of the palps, conducts collected 
material around the dorsal edges to the inner or apposed faces. 
On the dorsal margin of this latter surface, the stream is directed 
posteriorly, and across to the folds, on to which it is lifted, and 
is then started toward the mouth. The ciliation of the ventral 
margin is directed posteriorly to the tip, as usual. Posterior 
to the middle of each fold, but not deep down in the groove be- 
tween folds, is a ciliation upward to the dorsal margin, and 
material which happens to be carried to this margin, is thrown 
back again on to the folds, as in Schizotherus: but ventrally 
directed tracts, deep in the grooves, were not seen, perhaps be- 
cause when examinations were made, the presence of such tracts 
in Schizotherus and others, was not known. 
Visceral mass. There is a general ciliation over the walls of 
the visceral mass, collections being directed downward nearly 
to its ventral margin, then posteriorly to a position nearly 
opposite the base of the incurrent siphon, s. Here all material 
is brought to a vortex (fig. 13, v) a considerable part of it often 
being collected in a rotating ball before it falls, or is washed off 
by the incoming stream of water, to lodge on the mantle. 
Mantle. 'The mantle edges of Mya and its allies are united 
except for siphon and foot openings. The great expanse of its 
