740 



ZOOLOGY 



SECT. 



in addition to those formed by modification of foot ; they consist 

 of the two short tentacles situated on each side near the eye. 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 apposi- 

 tion, and not united. A valve, such 

 as has been described in Sepia, 

 accurs in most Decapoda and in 

 Nautilus, but is absent in the 

 Octopoda. Ckromatophores, 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 cham- 

 bers is occupied by the body of the animal ; the 'rest are filled 

 with gas. Perforating the middle of all the septa in succession is 



FIG. 650. Amphitretus pelagicus, 



an Octopod with the arms united by a 

 web. e. eyes ; /. funnel ; p. pouch in 

 the mantle. (From Cooke, after Hoyle.) 



Fio. 651. Shell of Spirula. A, outside view ; B, showing last chamber and position of 

 siphuncle ; C, in section, showing the septa and the course of the siphuncle ; D, shell broken 

 to show the convexity of the inner side of the septa ; E, portion of a septal neck. (After 

 Cooke.) 



. 



a spiral tube the siphuncle continuous with the centro-dorsal 

 region of the visceral prominence. In the course of its growth 



