300 MANUAL OF ZOOLOGY SECT. 



To compare such a cephalopod as a cuttle-fish or squid 

 with a fresh-water mussel or a snail, it is advisable to place 

 it in a position which it quite naturally assumes when not 

 swimming, with the head and its arms downwards and 

 the body sloping away from this upwards and backwards. 

 In 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 only external in the female argonaut and in Nautilus. 

 In the latter (Fig. 184) it is in the form of a flat spiral, the 

 interior 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 centro- 

 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 sep- 

 tum is formed closing the latter off from the cavity last 

 occupied. 



The Nautilus inhabits the coral reefs of the Pacific, at a 

 depth of a few fathoms. Of existing Dibranchiata, Spirula 

 (Fig. 185) alone has a shell comparable 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. 

 183 and 185, the relation of the soft parts to the shell is the 

 reverse of what obtains in Nautilus, the shell of Spirula curv- 

 ing backwards, that of Nautilus forwards. Moreover the 

 shell of Spirula is an internal structure, being almost com- 



