MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
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rnent in their protoplasm. Sections show no indication of the folding of the walls, 
such as is found in the gill of Yoldia , and I have seen no trace of the muscle fibers 
which I have described as being present in the interior of the plate in the latter form. 
Endothelial cells are easily demonstrated, lining the interior of the chitinous 
layers ( eh ). Lacunar tissue, if it exists, was not made out, as the walls of the plate 
are everywhere closely applied to each other, in sections, and only here and there cau 
very narrow spaces be seen between them. 
THE GILES OF ARCA PEXATA. 
If an entir e filament of the gill of this form be isolated, it will be seen to be made 
of a fully developed descending and ascending limb (Fig. 50, PI. lxxxvi). The upper 
end of the ascending limb in the filaments of both gills is free from the mantle or side 
of the visceral mass, as in Mytilus. It bears an errlarged triangular end plate which 
turns outward. The anterior and posterior faces of this end are ciliated and form a 
large patch which closely interlocks with those in the same position on neighboring 
filaments. 
The most striking fact in regard to these filaments is that the descending and 
ascending limbs are not separated from each other for about half their extent — the 
ventral half — but are connected by the continuation of their inner walls (cp, Pig. 50, 
PI. lxxxvi, and Fig. 66, PI. lxxxviii). 
There are no vascular connections between the filaments, but they are field together, 
as is usual in the genus Area , by ciliated disks (cd, Fig. 66) arranged in a row on the 
anterior and posterior faces of the filament throughout its entire extent. 
Fig. 66 represents a cross-section of four filameuts. Three of these are cut at a 
point above the union of the ascending and descending limbs ; a single one is cut lower 
down and passes through this connecting portion (cp). 
The sections show the filament to be thin from before backward and wide from 
side to side. The elongated cells of the frontal epithelium (/) are uniformly ciliated 
and extend back for some distance from the end, the cilia gradually becoming shorter, 
as do the cells which bear them. At a corresponding point on either side, there are 
three or four cells seen in each section, which are longer than usual and bear long 
cilia (Ifl). These are long ridges, or rows of cells, which are here cut in transverse 
section. They extend the whole length of the filament without any break, and their 
cilia do not serve to join contiguous filaments, but, as I believe, like the entirely similar 
rows in Yoldia (where there are two), are merely to prevent the main currents, carry- 
ing foreign particles, from entering between the filaments. Of course, currents of 
water do penetrate between the filaments to enter the water tubes of the gills, and 
thence proceed upward into the epibranchial chambers ; but in all cases, so far as I 
have observed, the cilia of the rows, or lines, point obliquely outward , presenting 
their ends to foreign particles and keeping them out on the ciliated frontal epithelium 
while not being thick and heavy enough to prevent water from entering. 
Peck (No. 16) has shown that in a position similar to this, though nearer the outer 
edge, in the gill filaments of Anodon, there was a single line of enlarged cells bearing 
very long cilia. Moreover, in Dreissena he found two lines of enlarged cells lying- 
together, with no cells between, but with the two lines of cilia distinct. These cells, 
one appearing on either side in each section, he called the latero-frontal cells, In the 
latter case he distinguished a first and second latero-frontal cell, 
