300 



MANUAL OF ZOOLOGY 



SECT. 



this position we distinguish antero-dorsal and postero-ventral 

 surfaces, oral and aboral extremities, and right and left 

 borders. 



A shell is present in nearly all Cephalopods, but is 

 external only in the female Argonaut and in Nautilus. In 

 the latter (Fig. 172) it has the form of a flat spiral, the in- 

 terior of which is divided by a series of transverse partitions 

 or septa into a corresponding series 

 of chambers. The last chamber 

 opens widely on the exterior, and 

 this alone lodges the body of the 

 animal, the remaining chambers 

 being filled with gas. Perforating 

 the middle of all the septa in 

 succession is a spiral tube the 

 siphuncle continuous with the cen- 

 tro-dorsal region of the visceral 

 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 occupied. 



Of existing Dibranchiata, Spirula 

 (Fig. 173) alone has a shell 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. Internally it is divided into chambers by 

 a series of septa, and these are perforated by a siphuncle. 

 Again, as will be seen by comparing Figs. 171 and 173, the 

 relation of the soft parts to the shell is the reverse of 

 what obtains in Nautilus, the shell of Spirula curving 



FIG. 173. Spirula peronii, 

 lateral view. d. terminal 

 sucker ; f. funnel ; j.l, s.- 

 projecting portions of the 

 shell, the internal part of 

 which is indicated by dotted 

 lines. (From Cooke.) 



