MOLLUSCA : CEPHALOPODA. 



345 



"arms " (fig. 143), which are generally provided with rows of 

 suckers. 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 

 radiating 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 Onychoteuthis] 

 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 en- 

 larged and club-shaped. In the 

 Pearly Nautilus the arms are 

 numerous 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 Mollnsca. 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 

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

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

 the o;her hand, unite posteriorly above the foot, and where they 

 coaleice, 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 

 beyoni the margin of the mantle, called the 'funnel,' or 

 ' infun^bulum/ as in the dibranchiate Cephalopoda " (Hux- 

 ley). \ 



The ^iouth leads into a buccal cavity, containing two 

 powerful! horny, or partially calcareous mandibles, working 



Fig. 143. Cephalopoda. Sepiola 

 Atlantica,o-nG of the Cuttle-fishes 

 (after Woodward). 



