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 (protoconcli) 

 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 



P 



Fi<;. (V.H. Amphitretus pelagicus, 



an Octopod with the arms united by a 

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

 the mantle. (From Cooke, after Hoyle.) 



I'M.. i'.!ii. 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.) 



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

 autilus, the shell of Spirula curving backwards (endoyastric 



the 

 Nautilus 



