hydro-dynamic law which governs its mode of self-propulsion when swimming, and possibly in some degree within the control of the animal--the latter is relieved of much of the weight of its shell. When the Nautilus is at the bottom of the sea its movement is like that of a snail crawling along upon the ground with its shell above it. The shell, in proportion to the size of the animal that inhabits it, is a heavy one, and unless it were rendered semi-buoyant, its owner's strength would be severely taxed by the effort to drag it along. By the means indicated this portable domicile is borne lightly above the body of the Nautilus, without in any way impeding its progress. [79] "At 100 fathoms the pressure exceeds 265 lbs. to the square inch. Empty bottles, securely corked, and sunk with weights beyond 100 fathoms, are always crushed. If filled with liquid the cork is driven in, and the liquid replaced by salt water; and in drawing the bottle up again the cork is returned to the neck of the bottle, generally in a reversed position."--Sir F. Beaufort, quoted by Dr. S. P. Woodward in his 'Manual of the Mollusca.' [Illustration: FIG. 32.--SHELL OF THE PEARLY NAUTILUS (_Nautilus pompilius_).] [Illustration: FIG. 33.--THE PEARLY NAUTILUS (_Nautilus pompilius_), AND SECTION OF ITS SHELL. _After Professor Owen._ _a a_, Partitions; _b b_, chambers; _b'_, the last-formed chamber, in which the animal lives; _c c_, the siphuncle; _d_, attaching muscle; _e e_, the hollow arms; _f f_, retractile tentacles; _g_, muscular disk, or foot; _h_, the eye; _i_, position of funnel.] The chambers are all connected by a membranous tube slightly coated with nacre, which is connected with a large sac in the body of the animal, near the heart, and passes through a circular orifice and a short projecting tube in the centre of each partition wall, till it ends in the smallest chamber at the inner extremity of the shell. Dean Buckland believed this "syphon" to be an hydraulic apparatus acting as a "fine adjustment" of the specific gravity of the shell, by admitting water within it when expanded, and excluding it when contracted. As it contains an artery and vein near its origin at the mantle, Professor Owen has regarded it as subservient to the maintenance of a low vitality in the vacated portion of the shell. Dr. Henry Woodward is of the opinion that, whilst in the early life of the Nautilus this siphuncle forms the main point of attachment between the animal and its shell, it is in the adult "simply an aborted embryonal organ whose function is now filled by the shell-muscles, but which in the more ancient and straight-shelled representatives of the group (the Orthoceratites) was not merely an embryonal but an important organ in the adult."