602 K. MITSUKURI. 
This would seem to be homologous with the trough-shaped 
chitinous structure in Nucula, but seems to be formed of the 
same fibres already referred to several times, which are 
found in the suspending membrane and other parts of the 
Nucula and Yoldia gills, and I cannot establish any con- 
nection between this bundle and the chitinous bars (Ah, fig. 
11) in each plate. The latter, when they reach the longi- 
tudinal bundle (s), make a bend and turn out again to enter 
the next plate in the series. In some sections I have ob- 
tained indications of a very thin layer of chitin beneath 
the fibrous bundle (s), which may, therefore, correspond to 
the fibres found 7 the trough of the framework in the 
Nucula gill (see above). If, however, this Y-shaped struc- 
ture is really homologous with the trough of the Nucula 
gill, it goes far in support of the view advanced above, that 
the chitinous framework is really made up of the fibrous 
tissue which is found in other parts, here cemented into 
one compact mass. In such a case fusion has gone further 
in Nucula than in Yoldia, and we see in the first genus the 
trough well united with the branches () in each plate. 
The plates (e, fig. 11) in Yoldia spread themselves upward 
instead of downward, as in Nucula. The chitinous bars 
(i), of which there are two in each plate, follow the curve 
of the plate and end rather bluntly about half way up, at 
the point @. ‘That the part from d to a@ corresponds to the 
lower inner edge of the Nucula plate (da, fig. 5) is shown 
by the characteristic rows of columnar cells having longer 
cilia than those found in other parts of the gill. There is 
another system of chitinous strnctures (ch, fig. 11). Many 
fine chitinous filaments come down together in a bundle on 
each side from the suspending membrane, and as soon as 
each bundle reaches the plate of its own side filaments 
spread themselves out like the frame of a fan over the whole 
plate. Several fibres sometimes proceed together, and then 
separating give the appearance of branching. They are 
found directly beneath the epithelial cells that cover the 
plate. The effect of this framework must be to keep the 
plate well spread out for the purpose of aération. I have 
not succeeded in obtaining any single section which shows 
the structure of the plate well, but from the comparison of a 
good many sections which I have made, I feel tolerably sure 
that the whole space between the epithelial surfaces is per- 
vaded by what Peck! calls “lacunar tissue” (fig. 12). Itis 
1 R. Holman Peck, “The Minute Structure of the Gills of Lamelli- 
branch Mollusea,”’ ‘ Quart. Journ. Micros, Sci.,’? 1877. 
