ABERRANT FORMS OF LAMELLIBRANCHIATE GILLS, 599 
seen in optical section. The surface of the irregularly rect- 
angular cells placed just inside these columnar cells in fig. 
5, ought therefore to be continuous with the outer edge of 
the columnar cells, but in order to avoid confusion is not so 
represented in the figure. - This is also the case with the 
cubical cells along the upper edge. The chitinous support 
(A, figs. 5 and 6) of the plate runs near the lower edge (fig. 
5) to its tip (a or 4, fig. 5), and is made up of two entirely 
separate parts (seen in cross-section in fig. 6) applied closely 
together. Owing to the shape of these parts there is, how- 
ever, a narrow oval space between them. ‘This space, as 
will be shown further on, is continuous with the space in the 
trough (s, fig. 5) of the stem. ‘The cells along the lower 
edge of the plate are columnar, and surround the chitinous 
support in a characteristic manner shown in fig. 6. Their 
surface outlines are irregularly rectangular, contrasting 
with the irregularly polygonal cells covering the rest of the 
plate. The branch (r, figs. 5 and 6) of the lower blood- 
channel (7) in the s¢em, is seen to be circular in cross-section 
and to bulge out the surface of the plate. These points are 
not, however, constant, as the vessel is sometimes constricted 
into more or less separate channels, while the amount of 
bulging seems to depend on the quantity of blood present. 
The remaining part of the plate (e, figs. 5 and 6) is flat and 
quite thin, enclosing a broad blood-channel between its two 
epithelial surfaces. It is here no doubt that the aération of 
blood is accomplished. ‘The cells of this part are cubical, as 
seen in fig. 6. Some of them send processes inward to join 
others from the opposite side. This gives a labyrinthine 
appearance to this part of the plate. The course of the 
blood is evidently from one blood-channel in the stem to the 
~other, through the space in the plate. For instance, the 
blood may start from the upper channel (0) in the stem, go 
to the broad flat part (e, fig. 5) of the plate where it gets 
aérated, then enter the branch (7), along its upper edge, and 
run up this to reach the lower blood-channel (v e) in the stem. 
This is, however, a purely hypothetical course. I have had 
no means of determining whether the blood goes from the 
upper to the lower channel or vice versd. 
The framework which supports the gill can be separated 
out by heating it in dilute caustic potash, as it is insoluble 
in weak acids and alkalies. It is stained by carmine and 
other colouring reagents. Whether it is really formed of 
chitin Ido not know, but as previous writers have described 
the substance as of that nature it will be convenient to use 
the term ‘‘ chitinous support” for the present. The frame- 
