It is thus the pearly nautilus floats under certain, circumstances on the surface of the water. The argonaut (Fig. 4), credited in poetry and fiction with this power, never floats on the surface, as was of old believed. It is simply a mundane cuttle-fish, whose two expanded arms are never used as sails, after the popularly supposed fashion, but are employed solely to secrete and attach to the body the false shell (Fig. 4, a) with which it is provided.
Among the two hundred odd living two-gilled cuttle-fishes, considerable diversity of external form may be seen; but the general type
already described is at the same time closely adhered to; and save in the case of the paper nautilus or argonaut, in which the characteristic shape of body is concealed by the shell, the cuttle-fish characters are readily apparent. The shell of the paper nautilus (Fig. 4, a) is termed "false" or "pedal," because it is not formed by the mantle, as all true shells are, but by the two expanded arms, as already mentioned. In its homology it therefore coincides with foot-secretions (such as the "beard" of the mussel), and not with the shells of its neighbors. The female argonaut alone possesses a "shell," the male (Fig. 4, c) being a diminutive creature, measuring only an inch or so in length. It is in the ranks of the two-gilled cuttle-fishes that we discover those phases of cuttle-fish life which most characteristically appeal to the popular mind. Thus, many species of two-gilled cuttles are eaten and considered dainties by foreign nations; it is from this group that the sepia color already mentioned is obtained; their internal shells gave us the "pounce" of long ago, and formed an article in the materia medica of by-gone days; and, lastly, it is in this group that the mythical and the real meet in the consideration of the giant cuttle-fishes which the myth and fiction of the past postulated, and which modern zoölogy numbers among: its realities.
The past history of the cuttle-fishes unites in itself a knowledge at once of their present position in the animal world and of their prog-