THE HISTOLOGICAL STRUCTURE OF THE GILLS, ETC. 99 
ard can be worked out that will hold true for the whole gill. 
However almost all the cells lining the ostia and the water 
tubes retain their cilia and are usually spherical in shape. 
Some even retain the columnar form and they are never flat- 
tened horizontally like those lining the inter-lamellar junc- 
.tions. ; 
The nuclei of these different types of epithelial cells, though 
varylng in size, in proportion to the size of the cell, are re- 
markably similar in all other respects so that the following 
description will hold for all of them. As mentioned before the 
nucleus is usually ellipsoid in form but is spherical in those 
cells which may be classified as cubical epithelium. The nu- 
clear membrane is always very distinct and stained heavily 
with haematoxylin. The neucloplasm is full of dark-staining 
eranules one of which is much larger and more distinct than 
the others and for this reason is called the nucleolus. The 
nucleo-plasm as a whole seems to take a darker stain than 
the cytoplasm but this appearance may be due to the optical 
effect of the numerous granules. 
~The nucleus of the single gland cell between the frontal and 
latero-frontal epithelium (fig. 4) is very large corresponding 
to the size of the cell, and at times has the appearence of two 
or even three nuclei clumped together (fig. 4-b.). When it 
-has this form the nuclear membranes are distinct but touch 
one another. These cells are therefore sometimes multi- 
nuclear. 
. The cells bearing cilia have a striated border at the-top to 
which the cilia are attached. This border is connected with 
the nucleus by a number of very fine fibrilis which run 
through the cell protoplasm giving it also a sort of striated 
appearance. Whether or not these fibrilis are continuous into 
the cilia, I was unable to ascertain. 
The chitinous rods supporting the gill fisraarits of the 
Lamellibranches are well known, and their structure was thor- 
oughly worked out by Peck (5). I have found that in the 
Najades the structure is similar to the marine species used by 
Peck in that the rods are two in number, one occurring on 
each. side of: the filament and similar to his in that they are 
not continuous througout the lamella but: are composed of: a 
