220 CONCHIFERA. 
these membranous leaves, which together compose 
the mantle, we find two pairs of other leaves, com- 
posed of radiating fibres of exquisite structure, 
attached to the body in one part, but elsewhere 
floating freely, so as to allow the surrounding water 
to bathe every part of their surface. These organs 
are the gills, and, on being examined with the 
microscope, show how beautifully their exquisite 
structure 1s contrived for the maintenance of a con- 
stant current of water over them. Lach of the 
four leaves is then seen to consist of a vast number 
of straight slender transparent filaments, evidently 
tubular, arranged side by side, so that 1,500 of 
them would be contained within the length of an 
inch. Strictly, however, these are but one filament, 
excessively long, bent upon itself again and again, 
at both the free and the attached end of the gill- 
leaf, throughout its whole extent. This repeated 
filament is armed on each of two opposite sides 
with a line of vibrating cilia, the two lines moving 
in contrary directions; by the action of which a 
current of water 1s made continually to flow up 
and down each of these delicate filaments; so that 
the blood which circulates in their interior (for they 
are doubtless blood-vessels) is continually exposed 
throughout this its long and tortuous course to the 
action of oxygen. 
Like all organic functions, the action of these 
cilia is not under the will of the animal. It is 
said that if, during life, a small portion of the gills 
be cut off, the motion of the cilia will convey the 
fragment swiftly away, with a smooth easy motion, 
through the surrounding fluid in a definite direc- 
tion. It does not even cease with the life of the 
animal, A specimen which I examined had been 
