2;8 



MOLLUSCA. 



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 pericardium, 



Fig. 248. Pearly Nautilus (Nautilus pompilius). a Mantle ; b Its dorsal fold ; 

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



and is continued through the entire length of the shell. The 

 position of the siphuncle is in the centre of each septum. 



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, surrounded 

 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 extremi- 

 ties, and shaped like the beak of a parrot, except 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 large crop, which in turn 

 conducts to a gizzard, and the intestine terminates at the base 

 of the funnel. On each side of the crop is a well-developed 

 liver. 



The heart is contained in a large cavity, divided into several 



