MOLLUSCA CEPHALOPODA. 



irn 



section through it: (Fig. 125.) Some, however, as the Jlr- 

 gonaut, or Paper Nautilus, have shells undivided hy par- 

 titions; and are accordingly termed unilocular or mono- 

 thalamous. The shell of the Argonaut is exceedingly thin, 

 iiand almost pellucid, probahly for the sake of lightness, for 

 it is intended to be used as a boat. For the purpose of ena- 

 bling the animal to avail itself of the impulses of the air, 

 while it is thus floating on the waters, nature has furnished 



126 



it with a thin membrane, 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 forw^ards 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 w^ell as acce- 

 lerate its motion. No sooner does the breeze freshen, and 

 the sea become ruffled, than 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 descrip- 

 tion, presents very curious phenomena. The animal at cer- 

 tain periods of its growth, finding itself cramped in the nar- 



* It must be confessed, however, that the habits of the Arg-onaut are still 

 ver)^ 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 pos- 

 session of by the Arg-onaut as a convenient habitation, which it can quit and 

 enter again at pleasure. 



