266 THE MECHANICAL FUNCTIONS. 



pulses of the air, while it is thus floating on the 

 waters, nature has furnished it with a thin mem- 

 brane, which she has attached to two of the 

 tentacula, so that it can be spread out like a sail 

 to catch the light winds which waft the animal 

 forwards on its course. While its diminutive 

 bark is thus scudding on the surface of the deep, 

 the assiduous navigator does not neglect to ply 

 its tentacula as oars on either side, to direct, as 

 well as accelerate its motion. No sooner does 

 the breeze freshen, and the sea become ruffled, 

 (han the animal hastens to take down its sail, 

 and quickly withdrawing its tentacula within its 

 shell, renders itself specifically heavier than the 

 water, and sinks immediately into more tranquil 

 regions beneath the surface.* 



The common Nautilus, which is provided with 

 a similar sailing apparatus, is an inhabitant of 

 a polythalamous shell (Fig. 126), of which Fig. 

 127 represents the section. The formation of 

 this, as well as of other shells of this description, 

 presents very curious phenomena. The animal 

 at certain periods of its growth, finding itself 

 cramped in the narrow part of the spire, draws 



* It must be confessed, however, that the habits of the Argo- 

 naut are still very imperfectly known. Considerable doubts are 

 entertained whether the shell it inhabits is formed by the animal 

 itself, or whether it is the production of some other, but unknown 

 species of Mollusca, and is merely taken possession of by the 

 Argonaut as a convenient habitation, which it can quit and enter 

 again at i)leasuie. 



