MORPHOLOGY OF LAMELLIBRAMCHIATE MOLLUSKS. 
417 
size throughout, a nd through them is evenly distributed a light yellow pigment, which 
is quite abundant and probably gives the red tint to the living gill. Immediately 
above the ventral edge, these sides expand laterally, leaving a large space (s), in which 
are numerous blood corpuscles. At the dorsal edge of the plate (d) there is also a 
marked widening of the bounding walls. The walls in the center of the plate are 
found in sections to be greatly convoluted (/), probably due to the contraction of mus- 
cular fibers ( mf ) contained within the space between the walls, to be described later. 
In these three regions described, the walls are connected with one another by 
numerous, branching, lacunar cells {lac), though they are less numerous in the central 
region (f). 
The entire interior of the plate is seen to be of one continuous blood space, 
with definitely constricted areas and enlarged channels. While blood corpuscles are 
naturally more abundant in these channels, they are found in all parts of the interior 
of the gill plate. 
Fig. 82 represents a section cut horizontally through the gill plates of one side 
in a plane indicated by the line b in Fig. 81. In this region, above the thickened 
ventral border, no chitinous framework appears. The outer edge of each plate is 
rounded and composed of short columnar cells, bearing very short, fine cilia on the 
extreme outer or frontal surface (e) ; but on the lateral sides of the rounded edges are 
narrow lines of cilia interlocking closely with each other ( cj ). At times, spaces ($) 
are found where the ciliation is absent, though these may perhaps be due to a mechan- 
ical tearing of one plate from another. The extent of these ciliated lines on the sides 
of the plates has been referred to above. The remaining parts of the plate walls are 
the same as already described, though less convoluted. In this section a part of the 
supporting membrane of the gill lias been cut across. Its surface epithelium (ep) is 
columnar, and many of the cells bear thick spines, perhaps bundles of cilia. The 
inner part of the membrane is made up chiefly of muscle bundles (m). 
Fig. 80 represents another horizontal section, passing close to the dorsal end of 
the plate (in the plane of c, Fig 81). Here the ciliated connecting lines or rows have 
disappeared and the epithelium is everywhere the same pigmented, indistinct kind 
above described for the main body of the plate. In this dorsal region, the lacunar 
cells (lac) are veiy numerous, and their processes, extending from one wall to another, 
are generally fine and thread-like. 
That the plates of one side of the gill structure are not continuous with those of 
the opposite side is evident from a section passing horizontally in the plane indicated by 
d in Fig. 81. Such a section is represented by Fig. 48, PI. lxxxvi. When the plates 
come together from either side on the median line (ml) they are not opposite each 
other, but on the contrary, break spaces. They are figured by Mitsukuri as being 
opposite each other in Nucula. The space a represents the interior of a plate, and b 
the space between two plates. The wall of one plate, then, runs over on the median 
line of the gills and becomes continuous with the wall of the next on the same side. 
Fibers have been described in this gill by Mitsukuri as running down into the plates 
from the supporting membrane above. He regarded them as chitinous structures, 
serving to keep the plate expanded for purposes of aeration. They are shown in Fig. 
79 and Fig. 48, mf, as being cut across more or less transversely. They are always 
closely applied to the inner face of one of the walls of the plate, but whether always 
to the anterior or posterior surface I do not know. A longitudinal, nearly vertical, 
F. 0. B. 1890—27 
