CILIARY MECHANISMS OF LAMELLIBRANCHS 641 
muddy waters of the Chesapeake are, in any given locality, com- 
paratively clear much of the time. It may be doubted if the 
Chesapeake ever becomes as muddy as the waters over the oyster 
fields near the mouth of the Mississippi; and yet even here, a 
large part of the time, they would be spoken of as clear. 
Secondly, in the contention that stomach contents sometimes 
contain a larger volume of sand than of diatoms, there appears no 
argument against the view that lamellibranchs feed only in com- 
paratively clear water, though, of course, they will receive some 
nourishment if there are any diatoms at all in the stomach. It 
will appear from the foregoing account of the palp functions that 
the volume of the material, and not its nature as possible food, 
determines whether or not it shall be taken into the stomach. 
If there is sand in the stomach of a lamellibranch, it has been 
taken in a very little at a time, and I know of no means by 
which this would be possible if very much at a time were 
received into the mantle chamber. Apparently, to adopt the 
view that feeding is possible in very muddy waters, is to make 
the seeming function of the outgoing cilia tracts quite meaning- 
less. And yet there remains the anomalous, not to say discon- 
certing case of the genus Macoma, of the Pacific, all the species 
of which I have found to possess outgoing tracts, and to be 
habitual sand eaters. It is difficult for me to believe that there 
is not some feature of the ciliation of these forms that has escaped 
my attention, which, if known, would explain this peculiar 
habit. These forms will be described subsequently. 
Visceral mass <A ciliation of the visceral mass could not be 
demonstrated in the specimens of Schizotherus examined, 
though possibly it is present, as in the majority of bivalves. 
The surface of the foot in all lamellibranchs seems to have lost 
its embryonic ciliation. . 
Venus mercenaria 
Gills. Material on outer and inner lamellae of both demi- 
branchs is moved to the free edges, and forward to the palps, as 
in Schizotherus. <A tract between the gills, at their bases, moves 
