THE ARGONAUT, OE PAPER NAUTILUS,
Is a kind of Poulpe, in which only six of the arms present the ordinary form, the other pair being expanded into broad, flat organs. It was supposed by the ancients, and, indeed, until very recently, that these expanded arms were used by the animal as sails; it was described as floating at the surface of the sea, with the back of the shell downwards, the six arms sticking into the water like so many oars, and the two broad members elevated to catch the breeze; but it is now known that the so-called sails are used to embrace the shell when the animal is swimming backwards, in the same way as its allies, and it also appears that it is by these arms that the shell is enlarged. The Argonaut is found in the Mediterranean.
THE NAUTILUS, OR PEARLY NAUTILUS,
(Nautilus Pompilius,)
Is a very different creature, and instead of the eight arms of the Argonaut has its head surrounded by numerous ringed and sheathed tentacles. It is remarkable for the structure of its shell, the cavity of which is divided into numerous chambers by transverse partitions; these chambers, of which the outermost alone is occupied by the animal, are filled with air, but a narrow tube passes through the whole of them, and communicates with the cavity of the body. By this arrangement the Nautilus is enabled to alter his specific gravity so as either to rise to the surface or sink to the bottom of the water. The few existing species of Nautilus are all found in the Indian and South Pacific Oceans.