It is an old belief, sanctioned by Aristotle, that the broad membranous expansions of the two arms, are hoisted by the animal as sails; and that in calm weather it sits in its boat-like shell, and floats over the smooth surface of the sea, steering and paddling with its other arms; and that, when danger threatens, it lowers its masts, and sinks beneath the waves. Oppian, in his “Halieutics,” poetically expresses his opinion that it served as a model for the man who first conceived the idea of constructing a ship, and embarking on the waters:— “If humble guess may probably divine, And trace th’ improvement to the first design, Some wight of prying search, who wond’ring stood When softer gales had smoothed the dimpled flood, Observed these careless swimmers floating move, And how each blast the easy sailor drove; Hence took the hint, hence formed th’ imperfect draught, And ship-like fish the future seaman taught. Then mortals tried the shelving hull to slope, To raise the mast, and twist the stronger rope, To fix the yards, let fly the crowded sails, Sweep through the curling waves, and court auspicious gales.” This pretty fable was exploded in 1837 by Captain Sander Rang, an officer of the French navy, and Port-captain at Algiers, who carefully followed up some experiments communicated to him by Mrs. Power, a French lady then residing at Messina; and the structure and purpose of the two flattened limbs is now clearly understood.[7] Instead of floating in its pleasure-boat over the sea, the argonaut ordinarily crawls along the bottom, carrying its shell above it, keel uppermost; and the broad extremities of the two arms are not hoisted as sails, nor allowed, when at rest, to dangle over the side of the “boat,” but are used as a kind of hood by which the animal retains the shell in its proper position, as a man bearing a load on his shoulders holds it with his hands. When it comes to the surface, or progresses by swimming instead of walking, it does so in the same manner as the octopus; namely, by the forcible expulsion of water from its funnel-like tube.[8] [Illustration: Fig. 1. The Paper Nautilus (_Argonauta argo_). The membrane is shewn partially retracted and the shell exposed.]