MOLLUSCA: CEPHALOPODA. 431 



The soft structures in the Pearly Nautilus may be divided 

 into a posterior, soft, membranous mass (metasoma), contain- 

 ing the viscera, and an anterior muscular division, comprising 

 the head (prosoma) ; the whole being contained in the capa- 

 cious outermost chamber (the body-chamber) of the shell, from 

 which the head can be protruded at will. The shell itself (fig. 

 233) is involuted and many-chambered, the animal being con- 



Fig. 233. Pearly Nautilus (Nautilius pompilius). a Mantle ; b Its dorsal fold ; 

 c Hood ; o Eye ; t Tentacles ; /Funnel. 



tained successively in each chamber, and retiring from it as its 

 size becomes sufficiently great to necessitate the acquisition of 

 more room. Each chamber, as the animal retires from it, is 

 walled off by a curved, nacreous septum ; the communication 

 between the chambers being still kept up by a membranous 

 tube or siphuncle, which opens at one extremity into the peri- 

 cardium, and is continued through the entire length of the 

 shell. The position of the siphuncle is in the centre of each 

 septum, but the siphuncle simply passes through the chambers, 

 without opening into them. 



Posteriorly the mantle of the Nautilus is very thin, but it is 

 much thicker in front, and forms a thick fold or collar sur- 

 rounding the head and its appendages. From the sides of the 

 head spring a great number of muscular prehensile processes 

 or "arms," which are annulated, but are not provided with 



by Prof. Owen in 1832. Since that time examples have been described by 

 Van der Hoeven, Vrolik, Valenciennes, Macdonald, &c. 



