MOLLUSCA CEPHALOPODA. 



•237 



are, like the Sepisp, provided with tentaoula at- 

 tached to the head. The shells of these animals 

 are often found to contain partitions dividing them 

 into a number of chambers ; hence they have been 

 termed camerated, or multilocular, or polythalamous 

 shells ; and sometimes the greater part of the shell 

 is contained within the body. The Spirula (Fig. 

 124) has an internal shell of this description, of 

 which the cellular structure and numerous par- 

 titions are rendered visible by making a section 

 through it (Fig. 125). The Nautilus Poynpilius is 

 the inhabitant of an external polythalamous shell, 

 (Fig. 126), the section of which is represented in 

 Fig. 127: but the shell of the Paper Nautilus, or 

 Argonaut, is unilocular, or monothalamous, being 

 undivided by partitions. 



The shell of the Argonaut is exceedingly thin, 

 and almost pellucid, probably for the sake of light- 

 ness, for it is intended to be used as a boat. For 

 the purpose of enabling the animal to avail itself 

 of the impulses of the air, while it is thus floating 

 on the waters, nature has furnished 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 

 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 



