MOLLUSCA: CEPHALOPODA. 271 



Each sucker, or ' acetabulum,' consists of a cup- shaped cavity, 

 the muscular fibres of which converge to the centre, where 

 there is a little muscular eminence or papilla. When the 

 sucker is applied to any surface, the contraction of the ra- 

 diating muscular fibres depresses the papilla so as to produce 

 a vacuum below it, and in this way each sucker acts most 

 efficiently as an adhesive organ. In some forms (Decapoda) 

 the base of the papilla, or piston, is surrounded by a horny 

 dentated ring, and in some others (as in OnyclioteutTiis) the 

 papillae are produced into long claws. In the Octopod Cuttle- 

 fishes there are only eight arms, and these are all nearly alike. 

 In the Decapod Cuttle-fishes there are ten arms, but two of 

 these called ' tentacles,' are much longer than the others 

 and bear suckers only at their extremities, which are enlarged 

 and club-shaped. In the Pearly Nautilus, the arms are nume- 

 rous, and are devoid of suckers. 



The arms are really produced by an extension of the mar- 

 gins of the ' foot,' or of the part corresponding to the foot of 

 the other Mollusca. The * antero-lateral parts of each side of 

 the foot extend forwards beyond the head, uniting with it and 

 with one another ; so that, at length, the mouth, from having 

 been situated, as usual, above the anterior margin of the foot, 

 comes to be placed in the midst of it. The two epipodia, on the 

 other hand, unite posteriorly above the foot, and where they 

 coalesce, give rise either to a folded muscular expansion, the 

 edges of which are simply in apposition, as in the Nautilus ; 

 or to an elongated flexible tube, the apex of which projects 

 beyond the margin of the mantle, called the " funnel," or " in- 

 fundibulum," as in the dibranchiate Cephalopoda.' (Huxley.) 



The mouth leads into a buccal cavity, containing two 

 powerful, horny or partially calcareous, mandibles, working 

 vertically like the beak of a bird ; together with an ' odonto- 

 phore ' or ' tongue,' the anterior part of which is sentient, 

 whilst the remainder is covered with recurved spines. The 

 buccal cavity conducts by an oesophagus into which salivary 

 glands pour their secretion to a stomach, from which an in- 

 testine is continued, with a neural flexure, to open on the 

 ventral surface of the animal at the base of the funnel. In 

 many cases there is also a special gland, called the ' ink-bag,' 

 for the secretion of an inky fluid, which the animal discharges 

 into the water, so as to enable it to escape when menaced or 

 pursued. The duct of the ink-bag opens at the base of the 

 funnel ; but this apparatus is entirely wanting in the Tetra- 

 branchiate Cephalopods, where in consequence of the presence 

 of an external shell, this means of defence is not needed. 



The respiratory organs are in the form of two or four plume- 



