CEPHALOPODA. 423 



are unrolled and flattened out, their resemblance to the foot of a 

 Gastropod is considerable, and Graham Kerr has suggested the 

 possibility of the animal being able actually to unroll them in life 

 and to use them as a foot (Fig. 337). In the Dibranchs the edges 

 of these folds are fused, so that the funnel is a complete tube 

 (Fig. 334), and in both Dibranchs and Nautilus the broad hinder 

 part of it is covered by the mantle-folds. 



On each side of the funnel there is in Decapods a peculiar sucker- 

 like arrangement by which the mantle-fold can be attached to the 

 funnel so as to close the general mantle-opening; it consists of a 

 smooth cartilaginous projection on the mantle, and a corresponding 



s 



Fio. 337. Side view of Nautilus pompilius extracted from the shell, the funnel has been opened 

 out, and the mantle-flap partly cut away (after Graham Kerr). /funnel ; e eye ; g gill ; h hood ; 

 m cut edge of mantle ; s siphuncle ; t tentaculiferous lobes. 



depression on the funnel. In the Odopoda the mantle-fold is fused 

 to the head anteriorly and laterally, so that the mantle opening is 

 much restricted. 



The deep part of the mantle-cavity is placed on the posterior 

 surface of the visceral sac, which in the natural position is the under 

 surface. It has in Dibranchs thick, muscular walls, and it contains 

 one or two pairs of bipectinate ctenidia, the median anus, the paired 

 renal openings, and the generative opening, Avhich is sometimes single 

 and sometimes paired. The regularly repeated contraction of the 

 mantle-muscles causes the expulsion through the funnel of the re- 

 spiratory water, which has been taken in through the mantle-opening, 

 and with it of the excrementitious (renal and anal) and generative 

 products. When the contraction is rapid and violent the jet from 

 the funnel causes the animal to shoot rapidly backwards ; on any 



