XII 



PHYLUM MOLLUSCA 



793 



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

 Decapoda and in Nautilus, but is absent in the Octopoda. 



FIG. 6St>. Argonauta argO, female, showing the relations of the animal to the shell in the living 

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

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

 Cooke, after Lacaze-Duthiers.) 



Ghromatophorcs, 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 



w*. i i!>0. Octopus lentus, male specimen, showing the structure of the hectocotylised arm 

 (h. a). (From Cooke, after Yen-ill.) 



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

 ing the middle of all the septa in succession is a spiral tube the 

 siphunclc continuous with the centro-dorsal region of the visceral 



