770 



ZOOLOGY 



SECT. 



ringed, sheathed tentacles, situated on lobes of the foot surrounding 

 the mouth, take the place of the arms, and suckers are not 

 present. In the males the spadix probably represents, functionally 

 at least, the hectocotylised arm of the Dibrauchiata. 



FIG. 699. Octopus lentus, male specimen, showing the structure of the hectocotylised arm 



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



In all the Dibranchiata the funnel is a complete tube. In the 

 Nautilus, on the other hand, as we have seen, the folds which form 

 the funnel have their edges merely in apposition, and not united. 

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

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



Chromatophores, similar to those of 

 Sepia, are universal in the Di- 

 branchiata 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 by the body of the 

 animal ; the rest are filled with gas. 

 Perforating the middle of all the 

 septa in succession is a spiral tube 

 flu- siphuncle continuous with 



FIG. 700. -Amphitretus peiagicus, t! . ie ^ro-dorsal region of the 

 an octwpod with the arms united by a visceral prominence. In the course 



the mantle ye (From Cooke' afte? n'oyle'.') f ^S growth the body of the 



Nautilus shifts forward at intervals 



into a newly-formed chamber, and a new septum is formed 

 closing the latter off from the cavity last occupied. It is onlv 

 after the last septum has been formed that the animal attains 

 sexual maturity. 



