90 PROFESSOR RAY LANKESTER. 
ance owing to the strong blue-green coloration of these 
cells and the colourless character of the surrounding sub- 
stance. 
When one of the branchial bars or filaments is isolated by 
teazing and examined with the microscope, it is found to present 
two rows on each face of these green-coloured secretion-cells, 
(fig. 13, gl.), whilst the rest of the filament is colourless. 
In transverse sections of the gill (fig. 11) the curiously com- 
plicated grouping of the branchial bars is seen and the position 
of the secretion-cells (g/.). The green-coloured secretion-cells 
are not confined entirely in the gills to the surface of the bars, 
but occur also irregularly upon the internal face of the gill 
lamella bounding the interlamellar water spacc (is.), where 
they are more irregularly scattered. 
It is difficult to decide absolutely that there is not a minute 
trace of blue-green pigment diffused through the protoplasm of 
all the epithelial cells, but there is no doubt that the pigment 
is concentrated in full intensity in the secretion-cells ; and [am 
inclined to regard the appearance of a very pale green tint 
diffused throughout the substance of the gill-filaments as due 
to optical conditions which allow the colour of those secretion- 
cells not in actual focus to be transferred by refraction and 
reflection to the surrounding colourless substance. 
The secretion-cells which are thus the actual seat of the 
pigment in the green gills of the Marennes Oyster are now for 
the first time shown to play that part. They are no peculiar 
possession of green Oysters, but occur in exactly the same form 
and position, but without colour, or of a slightly brown colour, 
in ordinary colourless (or brownish) Oyster’s gills. 
The secretion-cells furnish precisely that mechanism which 
we should expect to find in order that the blue pigment 
absorbed by the blood of the Oyster from the contents of its 
alimentary canal, namely, from ingested Navicula ostrearia, 
should be deposited at a particular spot on the animal’s body. 
These secretion-cells do not occur on other parts of the external 
surface of the Oyster; they are limited to the surface of the 
branchiz and to the adoral surface of the labial tentacles. 
