416 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
An examination of sections made through the gill plates in different regions will 
give a better idea of these points. Fig. 79 represents a section cutting the plate of the 
gill of Yoldia in a plane represented by the line a in Fig. 81. This section thus shows 
the structure of both the ventral and the dorsal borders of the plate as well as that of 
its interior regions. The thick ventral border is shown at the upper end of the figure. 
As pointed out by Mitsukuri for Nucula , this corresponds in structure to that which 
is usual in the outer edges of the gill filaments of other lamellibranchs. This portion 
of the gill is supported by a framework of the so-called chi tin (ch), which widens out 
at about the middle of its extent and makes the inclosed blood space a conspicuous 
channel at this place (6). The u chitin ” is here quite thick and gradually becomes 
thinner in either direction. 
The outer epithelium of the ventral edge is made up of closely packed columnar 
cells. Those at the extreme edge of the plate (e) bear peculiarly long and powerful 
cilia, which differ greatly in appearance from cilia in other regions of the ventral edge 
of the plate, or in fact in any other gill. In cross-section they are circular. The pen- 
cil cells, found frequently in other regions of the body in lamellibranchs, bear a stiff 
bristle which is found, by maceration, to be made up of several fused cilia. Whether 
or not these cilia may be of a similar structure I can not say, but it seems hardly 
necessary to suspect such a structure merely on account of their great size. Judging 
from their position and that of the other lines of cilia on the edges of the plate, these 
greatly developed cilia of the frontal cells are the ones which produce the rapid cur- 
rents in the water over the ventral surface of the gill. 
At some little distance inward from these frontal cells, the epithelium rises into 
a ridge on either side (r) and these bear a second row of cilia much shorter than the 
first, and very fine. They protrude laterally and outward, and their ends touch those 
of similar rows of cilia on contiguous plates. These rows, however, do not interlock 
with one another, and I believe that they serve simply to prevent currents of water, 
bearing food particles and other foreign bodies, from getting in between the plates, and 
not at all as a means of connecting neighboring plates. I think, also, that similar lines 
of cilia on the filaments of other forms, which will be noticed, serve the same purpose. 
A third line of cilia is borne by elongated cells, which do not, however, form a 
ridge, and is situated near the inner edge of the thickened, ventral edge of the plate (t). 
These cilia are longer than those of the second row and are also fine in appearance. 
In sections they appear bent outward toward the edge of the plate. 
All these rows of ciliated cells are sharply defined and the cells between them bear 
no cilia. Gland cells usually exist in the frontal region of all lamellibranch gill filaments 
or plates. If they appear in this region in the gill of Yoldia , it is in very small num- 
bers. I have frequently seen cells here which appeared much like gland cells, but I 
have not been able to decide positively that they were such. 
Lining the blood space inclosed by the chitinous layers is a perfectly distinct 
endothelium, represented in the figure. Hot only Avere the slightly elongated nuclei 
easily seen, but also the cell protoplasm flattened out over the chitin. There was no 
possibility of confounding these cells with the nucleated blood corpuscles seen in the 
blood space, as certain observers have been accused of doing in other forms. 
The thickened ventral edge, as seen in the section, is sharply separated from the 
remainder of the plate. This latter portion is made up of large epithelial cells, whose 
boundaries are not distinct and are seen but occasionally. These cells are of uniform 
