794 



ZOOLOGY 



SECT. 



prominence. In the course of its growth the body of the Nautilus 

 shifts forwards at intervals into a newly formed chamber, and a 



new septum is formed closing the 

 latter off from the cavity last oc- 

 cupied. It is only after the last 

 septum has been formed that the 

 animal attains sexual maturity. 



Of existing Dibranchiata, Spirula 

 alone has a shell (Fig. 692) com- 

 parable to that of Nautilus. The 

 shell of Spirula is of spiral form, 

 the turns of the spiral, however, 

 not being in close contact. In- 

 ternally it is divided into chambers 

 by a series of septa, and these are 

 perforated by a siphuncle. But 

 the initial chamber (protoconch) 

 instead of being, like the initial 

 chamber in Nautilus, similar to the 

 others though smaller, is dilated 

 into a spherical shape, constricted 

 off from the succeeding chamber, and has passing through it a 

 tube the prosiphon not continuous with the siphuncle. Again, 

 as will be seen by comparing Figs. 675 and 693, the relation of 



FIG. 691. Amphitretus pelagicus, 



an Octopod with the arms united by a 

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

 the mantle. (From Cooke, after Hoyle.) 



FIG. 692. 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 

 Cookc.) 



the soft parts to the shell is the reverse of what obtains in 

 Nautilus, the shell of Spirula curving backwards (endogastric 



