MOLLUSC A CEPHALOPODA. 265 



The other genera of Cephalopodous Mollusca 

 are, like the Sepiae, provided with tentacula at- 

 tached to the head. They comprehend animals 

 differing exceedingly in their size : some being 

 very large, but a great number very minute, 

 and even microscopic.* 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 potythalamous shells. The Spi- 

 rula (Fig. 124) is a shell of this description, of 

 which the cellular structure and numerous par- 

 titions are rendered visible by making a section 

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



124 



Argonaut, or Paper Nautilus, have shells undi- 

 vided by partitions; and are accordingly termed 

 unilocular, or monothalamous. The shell of the 

 Argonaut is exceedingly thin, and almost pellu- 

 cid, probably for the sake of lightness, for it is 

 intended to be used as a boat. For the purpose 

 of enabling the animal to avail itself of the im- 



* A particular account has been given of the shells of these 

 microscopic cephalopoda by M. D'Orbigny, in the Annales des 

 Sciences Naturelles ; vii. 96. 



