PHYLUM MOLLUSOA 



793 



A valve, such as has been described in Sepia, occurs in most 

 Decapoda ;ind in Nautilus, but is absent in the Octopoda. 



Fig. (5Sd.— Argonauta argo, female, showing the relations of the animal to the shell in the living 

 state, the arrow showing the direction of movement. /'. funnel ; ut. mouth, with jaws project- 

 ing ; eh. shell, with arms as seen through it ; ictt, webbed arm clasping the shell. (From 

 Cooke, after Lacaze-Duthiers.) 



Ohromatophorcs, similar to those of Sepia, are universal in the 

 Dibranchiata but absent in Nautilus. 



Shell. — The shell of Nautilus is the most complete and yet in 

 a certain sense the most primitive. As already stated, it is an 

 external shell of a spiral character, divided internally by septa 

 into a series of chambers. The last of the chambers is occupied 



Fin. li'.iO.— Octopus lentus, male specimen, showing the structure of the hectoootylised arm 

 (h. a). (From Cooke, after Verrill.) 



by the body of the animal ; the rest are filled with gas. Perforat- 

 ing the middle (jf all the septa in succession is a spiral tube — the 

 siphunde — continuous with the centro-dorsal region of the visceral 



