MOLLUSCA CEPHALOPODA. 



97 



the paper nautilus (Argonauta). The shell of the paper 

 nautilus is spiral, but without chambers. It is secreted 

 by the two webbed arms of the female, but is not attached 

 to the body of the animal. The male is much smaller 

 than the female, and is destitute of a shell. The shell of 

 the pearly nautilus is secreted by the mantle. It is a 

 spiral shell, and is divided into chambers by partitions 

 culled septa (Lat. septum, a partition), which are shaped 

 somewhat like saucers, and are pierced in the centre by a 



V 



Fio. 40. SECTION OP A PEARLY NAUTILUS, showing the chambers traversed by 

 the siphuucle. The animal occupies the last of these chambers. 



membranous siphuncle. As the animal increases in size, 

 it removes to a wider part of the shell, and walls off the 

 part previously occupied by a partition, always occupying 

 the last formed and largest chamber. Each chamber has 

 therefore been occupied by it in succession, and it still 

 keeps up a connection with the vacated chambers by the 

 siphuncle. 



There are many extinct families of cephalopods, allied 

 to the nautilus, whose remains have been deposited in the 

 15 E. G 



