THE HISTOLOGICAL STRUCTURE OF THE GILLS, ETC. 103 
this selection, since they must come in contact with these par- 
ticles before they reach the labial palps. It seems probable 
that this selection would be made in the groove of the inner 
gill at’ least. 
The cilia on the gills apparently strain the water entering 
the gills and probably this is the principal function of the 
long cilia on the latero-frontal epithelium. These cells are 
certainly located at a strategic position for this work, situ- 
ated as they are at the turns of the filaments. The ostia as 
well as the depressions between the filaments may be com- 
pared to streets; these cells might then serve as traffic police- 
men with their cilia stretched across the opening, permitting 
the water to enter by turning back dirt and foreign particles. 
The method of preparation of the sections, given before, 
should insure against plasmolysis of the tissues, but it is 
possible that in some cases this might have taken place. How- 
ever the uniform results in the tissues investigated makes it 
very unlikely. 
The training of the eye for observation of details is of 
course an important factor and due to lack of this at the start 
of the work some important details may have been overlooked. 
The measurements were taken with standard equipment and 
it is hardly possible that it was at fault. 
The drawings are of course idealized somewhat since all 
the details of structure would lead to confusion. 
VI.— BIBLIOGRAPHY. 
(1) Allen, W. R. 
714. The Food and Feeding Habits of Freshwater 
Mussels. Biol. Bulletin, Vol. XXVII pp. 127-139. 
(2) Allen, W. R. 
21, Studies of the Biology of Freshwater Mussels. 
Biol. Bulletin, Vol. XL pp. 210-241. 
(3) Grier, N. M. 
"19. Morphological Features of Certain Mussel Shells 
found in Lake Erie, Compared with Those of the 
