CEPHALOPODA. - 285 
The order Zetrabranchiata contains the pearly nautilus 
(Nautilus) and a number of extinct allies. The nephridia 
and ctenidia are reduplicated, hence there are two pairs. 
The tentacles have no suckers and there is a large external 
shell. The shell of the pearly nautilus is chambered. The 
animal inhabits the last chamber. A median hole through 
each septum transmits a long process of the body called 
the s¢phuncle. 
Fig. 201.—LATERAL VIEW OF A NAUTILUS IN ITS SHELL. 
(After OWEN.) 
Note the hollow chambered shell and the numerous short tentacles. 
0, eye; g, siphuncle ; ¢, tentacles; 7, mantle; Z, hood ; e, siphon. 
The Dibranchiata contains the cuttles, squids and the 
octopus. In all there are two ctenidia and nephridia and 
the shell is either internal or absent. Octopus has only 
eight tentacles and no shell. The paper Nautilus (A7gonauta) 
also has only eight arms, and the female secretes a thin delicate 
shell. It is used to carry the eggs and is unchambered. 
The ammonites are fossil forms allied to Vautelus, whilst 
the belemnites are fossil Dibranchiata. They occur in great 
