422 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
three filaments (fil) in a region where the projections occur. The conical points, which 
I would designate as ciliated spurs (cs), protrude abruptly into the water tube between 
the lamellae of the gill. 
A section passing transversely through the filaments, and in the long axis of a 
few of these ciliated spurs, is shown in Fig. 86, at cs. Their walls are seen to be 
merely a continuation of the single-cell-layered wall of the filament. They are closely 
interlocked by cilia. Their interior is filled up with a solid mass of cells whose nature 
I have not positively determined. In macerated specimens, they appear to be much 
elongated in the long axis of the spur, but sections failed to show this satisfactorily. 
These projecting spurs are very conspicuous in any macerated preparation of the gill 
of Pecten. 
GILL OF VENUS MERCENARIA. 
This folded gill is, in many ways, more complicated than that of Pecten. The fila- 
ments have a vascular connection with one another at their inner edges, spaces being 
left here and there to allow water to enter the water tube of the gill (Fig. 70, w, PI. 
lxxxviii). The great primary folds, marked off by the primary reentering angles (raj), 
include about seventy filaments. These folds are sometimes divided, however, by 
a second reentering angle (m 2 ), into two secondary folds. There is no connection 
between the gill lamellae at this point. At the primary reentering angle there is a 
partition between lamellae, consisting principally of muscle bundles ( runs ). Between 
these exists a blood channel (bv). From these channels is sometimes given off into 
the water tube on either side of the partition a huge blood sinus (pbs). These are 
so large as to almost completely fill the space between the lamellae. 
Similar blood sinuses appear in other cases from an enlargement of the filament 
at the angle fbs , and may also be so great as to entirely fill the water tube. In re- 
gions where this condition is present, blood sinuses from the partitions are very small 
or absent. 
Venus being a large form and quite active, must require a considerable amount 
of food and at the same time sufficient aeration of blood. The narrow filaments, not 
sufficiently large to contain much blood, seem to have been specialized for procuring 
food. These large blood sinuses may have been developed to provide for the dimin- 
ished aeration in the filaments. Though they nearly fill the water tube, their thin 
walls are surrounded by water on all sides, probably in quantities sufficient for the 
purpose. 
THE GILL OF OSTREA VIRGINIANA. 
The gill of this very degenerate form is probably the most complex in the group. 
The lamellae are thrown into a number of folds between each of the thick cross- 
partitions. The filaments may be seen almost everywhere in section (Fig. 76, PI. xc) 
to have a vascular connection with one another at their inner edges. This is the case 
with all the filaments in the fold marked 2 in the figure to which reference has been 
made. Here the entire inner space of the fold is a blood sinus, with which the blood 
channel of every filament is continuous. In fold 1, three filaments at its outer extrem- 
ity are connected by their inner edges. In fold 3, four filaments on the left are thus 
connected, and this common sinus is seen to be continuous with another, close under 
the wall of the water tube of the gill ( wt ). 
