Page:Natural History, Mollusca.djvu/78

 thoroughly understood. Milne-Edwards, whose opinion is entitled to the highest respect, gives the following explanation of it:—"The skin of these animals is furnished with a number of differently-coloured spots, which alternately appear and disappear, and if a portion is put under a microscope, it may be perceived that these changes depend on the contraction of small vesicles filled with a coloured liquid, which reach from the surface of the skin to a considerable depth. When one of these spots appears, the liquid, corresponding here to the pigment in the other case, is propelled towards the superficial part of the vesicle, and there displays itself; whilst during its disappearance it is forced into the deeper parts by the contraction of this superficial point itself, which then becomes almost invisible."

The lingering rudiment of a vertebrate skeleton in these animals has been already noticed; their body encloses, however, a solid support of quite another nature, which represents the true shell so characteristic of Mollusca generally. Within the substance of the mantle, if we slit it up along the line of the back, we find an oblong cavity, within which lies loose, and unconnected with it a large plate, horny in some species and shelly in others. The pen of the common Squid (Loligo vulgaris) is of the former texture; the substance called cuttle-bone, so often found on sandy beaches, is of the latter; but both are strictly analogous to