PHYLUM MOLLUSCA 



781 



Fie. 678.— Nautilus pompilius, 

 spadix of full-grown male, seen 

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

 modified tentacles ; 1, withdrawn 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.) 



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. 677, 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 com- 

 pletely closed tube, the edges of its 



right and left moieties being simply 



in apposition posteriorly without being 



united together. Near the oral end 



is a large, somewhat triangular valve 



arranged like that of Sepia. 



There is an internal skeleton of 



cartilage (Fig. 679), 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. 

 Ventral ly and posteriorly the mantle en- 

 closes a large mantle-cavity (Fig. 680), cor- 

 responding 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 

 of the ctenidia of each side is a small knob- 

 like elevation, the oral osphradium (ant. os.), 1 

 and behind the bases of the more aborally 

 situated pair are two compressed, bilobed 

 projections, more or less completely united 

 in the middle so as to form a transverse 

 ridge ; these are the aboral osphradia (p. os.). 



In the middle line of the mantle-cavity is the anus (an.), a large 



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 relafivt 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. 



VOL. I 3 D 



Fu;. 07'.).— Nautilus pom 

 pilius, cartilaginous in 



ternal skeleton. 

 Keferstein.) 



(After 



