e. 



: 



xii PHYLUM MOLLUSCA 729 



themselves are in the adult male enormously thickened, and 

 the outer surface of the most posterior (3) is covered with regularly 

 arranged rows of minute pits. A fourth tentacle much smaller 

 than the others is closely applied to the outer surface of the 

 organ. In the internal lateral lobe, right or 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-spa (Ji> .-. 



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 

 ridge, close to the inner posterior lobe on 

 either side, of an area thickly beset with 

 elicate membranous ridges. 



On the posterior side of the head is a 

 unnel corresponding with that of Sepia, 

 but extending further forwards ; this, how- 

 ever, does not form a complete 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, 



An internal skeleton of cartilage, as in 

 Sepia, protects "the nerve-centres and sup- FIG. wo. Nautilus pom 



*f , , P,I p ., /T-C ^A/k\ pilius, cartilaginous in- 



ports the basal parts Of the foot (Fig. 640). ternal skeleton. (After 



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. 641) corresponding to that of Sepia. In this are lodged 

 two pairs of ctenidia (den.), 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 

 ospliradium (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 (post. os.). In the middle 

 line of the mantle-cavity is the anus (an.), a large aperture with 

 minutely lobed margin, situated on a slight elevation, but by no 



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. 



