598 K. MITSUKURI. 
the paired plates (e. ¢,) are seen to proceed. The coloured 
part at the bottom represents the complex chitinous frame- 
work. The membrane (4 77 /) is made up of fibrous 
tissue, the bundles of which this is composed crossing each 
other in many directions. Its free surfaces are covered 
with columnar epithelium. The stem consists mostly of a 
solid mass of large irregular cells with rather large nuclei. 
There are, I am almost certain, ¢wo blood-channels exca- 
vated through it; a lower larger (7), and an upper smaller 
(0). The latter seems to be in connection with a free space 
(g.) found often in sections of the suspending membrane. 
The large channel (z) sends a branch (r) into each plate. 
The fibrous tissue found in the upper membrane dips down 
into this part at regular intervals, viz. between every branch 
(r) of the lower blood channel (x). How these fibres end 
below, when they reach the chitinous framework, I have 
not been able to make out. A few fibres (w) are sent down 
into the plate a little above the blood-channel (7), and 
gradually approach and finally touch the latter near its 
lower end. A few more fibres (¢) are seen along the upper 
edge of the plate. Exactly what this fibrous tissue is 1 am 
unable to make out, but it seems to be some sort of tough 
connective tissue, with perhaps muscular fibres more or less 
intermixed. ‘That it is very tough and serves as a support 
to the whole structure is seen by the fact that the fibres 
often stick out beyond the broken edge of the soft tissues. 
The trough of the chitinous framework is seen at s,in 
cross-section. It extends along the whole length of the gill 
and sends out two branches into each plate. I have ob- 
tained the appearances, in some sections, of a bundle of 
fibrous tissue running in it and filling it. The framework 
will be described more fully further on. The plates (e), the 
proper respiratory organs, are comparatively speaking very 
broad and quite thin, and hang down from the solid part of 
the gill. The epithelium of the plates which is repre- 
sented in the figure as ending abruptly at the edges ¢ d and 
Jj d, turns at a right angle at these lines to cover the stem, 
and is soon reflected outwards again to form the epithelium 
of the next plate in the series. This is evident from an 
inspection of fig. 8. Hach plate may be said to be simply 
an enormously widened blood-channel (fig. 6), and as the 
blcod is necessarily spread out in a thin layer over a large 
area, the purposes of aération must be admirably served. 
The columnar epithelial cells seen at ad, fig. 5, are very 
characteristic of the plates under a microscope, and are the 
cells (dq, fig. 6) around the chitinous bars (A, figs. 9 and 6) 
ae 
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