CILIARY MECHANISMS OF LAMELLIBRANCHS 653 
Mytilus edulis and M. californiana 
Cilia tracts on most of the organs of Mytilus and its allies are 
clearly defined. Figures 18 and 19 are drawn from M. edulis, 
but M. ecaliforniana is almost exactly similar in anatomy and 
ciliation, though it is of immense size, one individual, taken from 
Puget Sound, having a lengthof 19 cm., or 73 inches, and a weight 
of one pound and nine ounces, others, of several clusters having 
about the same size. 
Gills. The movement by cilia on all lamellae, is to the free 
margin. The outer lamella of the outer demibranch is not united 
with the mantle, nor the inner lamella of the inner demibranch 
with the visceral mass, but at the bases of both of these is an 
anteriorly directed stream toward the palps. In both species 
there seems to be no tract between the bases of contiguous 
demibranchs. 
Palps. The apposed faces, bearing the folds, have wide dorsal 
and narrow ventral margins. The relation of palps to each 
other and to the anterior ends of the demibranchs (0g and 7g) 
is represented in figure 19, a view of the organs as seen from 
below. The anterior palp (ap) is folded forward. In the 
center of the figure, opposite the gill, is the groove between palps 
(the proximal oral groove pg) leading to the right toward the 
mouth. The mouth opening is hidden by the apposition of the 
palps over it. Material collected on the backs of the palps, is 
brought over their dorsal edges, and carried diagonally across 
the dorsal margin to the folds. If very small in amount, it 
crosses them to the oral groove, and, deep down in this, is carried 
toward, and into the mouth. Perhaps no part of the ciliary 
mechanism is so difficult to demonstrate in lamellibranchs in 
general as this proximal oral groove; but various materials have 
repeatedly been sent through it, and have been seen to enter 
the mouth, in all members of this group which have been exam- 
ined, as well as in several others. The secret of success is the 
employment of very minute quantities. Not much is required 
to extend out of the groove far enough to come in contact with, 
and to be transferred to, outgoing streams, shown here near the 
