Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/247

 Rh moved and fed at the bottom of deep seas, and at other times rose and floated upon the surface.

The Nautili (see Pl. 31. Fig. 1. and Pl. 32. Figs. 1. 2.) constitute a natural genus of spiral discoidal shells divided internally into a series of chambers that are separated from each other by a transverse plates; these plates are perforated by a tube or siphon, passing through the transverse plates, either at their centre, or towards their internal margin. (Pl. 1. Fig. 31. Pl., 32. Fig. 2. and Bl. 33.)

The external open chamber is, very large, and forms the receptacle of the body of the animal. The internal close chambers are void, and have no communication with the outer chamber, excepting for the passage of a membranous tube, which descends through an aperture in each plate to the innermost extremity of the shell, (Pl. 31, y. y. a. b. c. d. e. and Pl. 32, a. b. d. e. f.) These air chambers are destined to counterbalance the weight of the increasing body and shell of the animal, and thereby to render the whole so nearly of the weight of water, that the difference arising from the membrane of the siphuncle being either empty, or filled with a fluid, may cause the mass to swim or sink.