CILIARY MECHANISMS OF LAMELLIBRANCHS 685 
removal is its function, is not to be questioned. What seems 
to be an homologous structure, was found in B. pacifica. 
Barnea pacifica Dall. 
This beautiful form was studied at San Diego Bay, and was 
taken from stiff clay, the burrows being four or five inches in 
depth. In relation of siphon to body, it much resembles Zir- 
faea, previously described. As in that form, the gills extend 
posteriorly far into the siphon (fig. 58), there forming the septum 
between incurrent and excurrent tubes. 
Gills. Currents on all lamellae are downward to demibranch 
margins, where, as in Zirfaea, the groove leading to the palps 
possesses walls so high that they arch over and convert it into a 
closed tube, opening to receive and discharge collections (fig. 59). 
There are anteriorly directed streams between the bases of the 
demibranchs. 
Palps. Here, again, these organs are relatively extensive. 
The ciliation of folds and margins is precisely similar to that of 
Pholadidea penita, except that here a very narrow tract, lying 
along the upper ends of the folds, lashes toward the oral groove, 
but is so narrow that collections from the dorsal margin cross it 
on the way to the folds, with little influence from it. It is safe 
to say that there are no ventrally directed tracts on the palp folds. 
Visceral mass. Figure 60 represents the visceral mass from 
the right side, after mantle, palps, and gill have been removed. 
Ventralward there is a disk which contains some muscle tissue, 
but the sexual mass extends to the disk surface, and no foot 
remains, unless a few scattered muscles are the vestiges of it. 
Figure 61 is a ventral view of the visceral mass. In the last 
three figures there is shown a thin, nearly transparent struc- 
ture (cm) with a slight concavity on its ventral surface, which 
is evidently the collecting membrane so greatly developed in 
B. costata. When a comparison is made with figure 57, the 
course taken here by cilia currents is very suggestive. It will 
-be observed that these currents are directed to a region on the 
side, near the middle of the body, whence they course to the 
