MOKFHOLQGY OF LAMELLIBKANCHIATE MOLLUSKS. 
423 
Very frequently, however, openings appear between the filaments. In fold 3 this 
is the case with the majority of the filaments in the plane of this section. Here water 
may enter between the filaments into the interior of the fold and thence into the water 
tube of the gill (wt). 
A filament has much the same structure as in Pecten, excepting that the chitinous 
layer is abruptly thickened a short distance from its outer edge (ch). There is no con- 
stant septum across the blood channel of the filament. I believe that an endothelium 
is present, but can not be positive. 
The surface epithelium is of columnar, ciliated cells in the frontal region (/,). At 
its lateral edges appear gland cells in the usual position (glc). They are seldom seen 
on the inner edge of the filament in 0. virginiana, described by Lankester (No. 9) as 
being as numerous here as at the side of the frontal region in 0. edulis. The gland 
cells are not spherical, but much elongated in the specimens which I have examined. 
As in Pecten , certain cells crowd together a little distance inward from the gland 
cells, to form the compound straining line. The ciliated lines (l/l) do not interlock, but 
touch each other and form a barrier to foreign particles which might otherwise be 
carried into the water tube. 
The filament at the reentering angle (ra) is very greatly modified. It is essen- 
tially similar, whether or not it is extended across to the opposite lamella of the gill 
to form the thick partitions between the water tubes. The structures showing it to be 
morphologically a filament are the ciliated frontal epithelium (/ 2 ), the chitinous layer, 
here modified into two extremely large and thick rods (c/?, 2 ), and, in most cases, a 
blood channel interior to these. 
The filaments next these at the angle of the fold are broadened and shortened ; 
their chitinous layer is much enlarged and has become rod-like on either side. In 
fact, they offer a transitional stage between the ordinary filament and the extremely 
modified filament at the reentering angle of the folds. Such a transitional filament 
is that represented at tf. 
The partitions connecting the gill lamellae are lined by an epithelium of cuboidal 
cells ( ep ), continuous with that which lines the remaining walls of the water tube. 
The interior of the partition consists of many longitudinal and cross muscle fibers 
( mns ), and between these often lie masses of large circular cells (pc), which I have 
only seen in the partition. A considerable portion of the partition is often occupied 
by blood spaces (bv). 
Muscle fibers also occur in masses interior to those reentering angle filaments not 
connected with a partition. They seem to be extended in the long axis of the gill. 
