308 MANUAL OF ZOOLOGY. 



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

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

 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. 



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

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



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 

 cups or suckers. In the centre of the head is the mouth, sur- 

 rounded by a circular fleshy lip, external to which is a series 

 of labial processes. The mouth opens into a buccal cavity, 

 armed with two horny mandibles, partially calcified towards 

 their extremities, and shaped like the beak of a parrot, ex- 

 cept that the under mandible is the longest. There is 

 also a " tongue," which is fleshy and sentient in front, but is 

 armed with recurved teeth behind. The gullet opens into a 



