these membranous leaves, which together compose the mantle, we find two pairs of other leaves, composed 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 is contrived for the maintenance of a constant current of water over them. Each 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 is 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 direction. It does not even cease with the life of the animal. A specimen which I examined had been