CEPHALOPODA. 



28:; 



The order Tetrabratichiata contains the pearly nautilus 

 ^Nautilus) and a number of extinct allies. The nephridia 

 md ctenidia are reduplicated, hence there are two pairs. 

 The tentacles have no suckers and there is a large external 

 >hell. The shell of the pearly nautilus is chambered. 1 he 

 inimal inhabits the last chamber. A median hole through 

 3ach septum transmits a long process of the body called 

 he siphimde. 



Fig. 201. — Lateral View of a Nautilus in its Shell. 



(After Owen.) 



Note the hollow chambered shell and the numerous short tentacles. 

 0-1 ^ys ; S-> siphuncle ; t, tentacles ; vt, mantle ; /, hood ; e, siphon. 



The Dibrarichiata 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 {Argo?iauta) 

 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 Nautilus^ whilst 

 the belemnites are fossil Dibrarichiata. They occur in great 



