424 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
GENERAL CONCLUSIONS IN REGARD TO THE GILLS. 
1. The epithelium of the ventral edge .of the gill plates in forms like JYucula, 
Yoldia, and Solenomya , and of the outer edge of the filament in all other lamelli- 
branchs — morphologically the same thing — is specialized by the possession of 
“clritin,” large ciliated cells, and gland cells. This of course does not apply to the 
deep-sea forms with a septum and no gill. This probably served in the ancestors of 
the group for the procuring of food as well as for aeration. However large the palps 
may be, they are certainly not sufficient in Yoldia for obtaining food. The gills 
here, and probably in other forms with plate gills, are extremely active in collecting 
food. Besides Yoldia , which I have observed, this function is evident in the others, 
from the presence of gland cells. 
2. The single row of latero-frontal or straining cells, described by Peck in Anodon, 
and the two single cell rows of Dreissena , are not present in any of the marine forms 
which I have examined, with the exception of Mytilus. Here there is a single row of 
straining cells, and interiorly from these a row of many closely crowded, ciliated cells, 
which I liave called the cells of the compound straining line. This arrangement, I 
believe, will be shown to be the most usual one in lamellibranch gills. 
There maybe one or two of these lines on each side of a filament (including the 
simple lines of straining cells when they occur). I have never seen more than two. 
In Solenomya and Anomia there seems to be no such specialization, all the cells of 
the outer edge of the filament being of much the same size and equally ciliated. 
I believe the function of these cells to be that of preventing foreign bodies in the 
water from entering between the plates or filaments of the gill, by means of their large 
cilia, while allowing currents of water to do so. 
3. Gland cells were present at the sides of the frontal region, from one to three 
appearing in a section on either side. The forms in which they were seen in this posi- 
tion were Solenomya , Area, Mytilus , Pecten, Anodon , Venus , Mya, Anomia , and Ostrea. 
I was not certain about the matter in Yoldia , though I believe that I have seen them 
in the gill of that form; and the fact that I have seen carmine grains firmly cemented 
into considerable masses on the gill of the living animal confirms this belief. 
In Mytilus they generally appeared immediately interior to the outer row of latero- 
frontal cells. Lankester (No. 9) has described gland cells as occurring near the inner 
edge of the filament in Ostrea edulis quite as constantly as at the sides of the frontal 
region. In Ostrea virginiana I have found them present in this region in certain 
specimens, but not in all. When they occur they seem to be always nearly spherical, 
those outward being constantly elongated. 
In some instances, gland cells occur on the inner edge of the filament, most 
noticeably in Anomia, where they are extremely abundant, very much more so than in 
the outer edge (Fig. 85). 
4. I believe that an endothelial lining of the blood cavity of the filament or plate 
between the chitinous layers will be demonstrated in the majority, at least, of the 
Lamellibranchiata. I am quite certain of having observed it in Yoldia , Pecten , Myti- 
lus , Anodon , Venus and probably in Area and Ostrea. It is especially well shown in 
Pecten and Yoldia. 
