56 



CEPHALOPODA. 



{Argonauta) and the Pearly Xautilus {Nautilus ijompilius) ; 

 but not only is the structure of the animal different in each 

 of these, but the nature of the shell itself is entirely different. 

 The shell of the Argonaut is involuted, but is not divided 



Fig. 447.— a, Internal skeleton of Sepia ornafa; h. Pen of Histiofevtliis BuneUiana; c, Shell 

 (" phragmacono ") of Spi nda fruijilis ; d, Animal of Spirula Fcronii. 



into chambers, and it is secreted by the webbed extrem- 

 ities of two of the dorsal arms of the female. The arms 

 are bent backwards, so as to allow the animal to live 

 in the shell, but there is in reality no organic connection 

 between the shell and the body of the animal. In fact, the 

 sliell of the Argonaut, being confined to the female, and 

 serving by its empty apex as a receptacle for the ova, may 

 be looked upon as a " nidamental shell," or, as it is secreted 

 by a modified portion of the foot, it may more properly be 

 regarded as a " pedal shell." The shell of the Pearly Nauti- 

 lus (fig. 448), on the other hand, is a true palhal shell, and 

 is secreted by the body of the animal, to which it is organi- 

 cally connected. It is involuted, but it differs from the 

 shell of the Argonaut in being divided into a series of cham- 

 bers by shelly partitions or septa, which are pierced by a 



