XII 



PHYLUM MOLLUSCA 



750 



- 2 



left as the case may be, opposite that bearing the spadix, the 



latter is represented by a group of four tentacles forming what is 



termed the anti-spadix. 



A further difference between the male 



and the female with regard to the foot 



is the presence in the latter, but not in 



the former, on the inner surface of the 



outer ring, close to the inner posterior 



lobe on either side, of an area thickly 



beset with delicate membranous ridges 



(organ of Valenciennes, Fig. 686, val.). 

 On the posterior side of the head is a 



funnel corresponding with that of Sepia, 



but extending further forwards ; this, 



however, does not form a completely 



closed tube, the edges of its right and 



left moieties being simply in apposition 



posteriorly without being united to- 



gether. Near the oral end is a large, 



somewhat triangular valve arranged like 



that of Sepia. 



There is an internal skeleton of carti- 



lage (Fig. 688), as in Sepia, but its 



relationships with the nerve-ganglia 



are much less intimate in the case of Nautilus than in that of 



Sepia. 



Mantle and Mantle - cavity. - - The 

 mantle is produced around the head into a 

 free flap, longer and looser than the mantle- 

 flap of Sepia. Dorsally this splits into two 

 layers, reflected over the convexity of the 

 shell, which fits into a hollow behind the 

 hood. Ventrally and posteriorly the mantle 

 encloses a large mantle-cavity (Fig. 689), 

 corresponding to that of Sepia. In this are 

 lodged two pairs of ctenidia (cten.), having 

 the same general structure as the single 

 pair present in Sepia. Between the bases 

 o f fa e ctenidia of each side is a small knob- 



FIG.C87. Nautilus pompilius , 



spadix of full-grown male, seen 

 from the outer side. 1, 2, 3, 4, 

 mortified tentacles ; 7, withdrawn 

 into its sheath, its position and 

 shape indicated by the dotted 

 line ; 3, the flattened tentacle 

 with the rows of minute cavities ; 

 x, patch of modified integument. 

 Two-thirds of the natural size. 

 (After Haswell.) 



pilius, Cartilaginous in- 77- \i 



temai skeleton. (After like elevation, the oral ospnradmm (ant. os.), L 



and behind the bases of the more aborallv 

 situated pair are two compressed, bilobed projections, more or 



1 As in Sepia, it is convenient to use the term oral for parts towards the 

 mouth end, and aboral for those situated towards the opposite extremity, the 

 same terms being also used to indicate relative position of different parts. The 

 relative position of the parts is, however, for the sake of simplicity given here 

 as they lie when the mantle-cavity is opened by turning back its thin postero- 

 ventral wall. 



