MOLLUSCA : CEPHALOPODA. 



419 



ing only an internal skeleton, and this often a rudimentary one ; 

 but the Argonaut (Paper Nautilus), and the Pearly Nautilus, 

 are protected with an external shell, though the nature of this 

 is extremely different in the two forms. 



The integument in the Cuttle-fishes is provided with nu- 

 merous mobile cells, containing pigment-granules of different 

 colours, and termed "chromatophores." By means of these 

 many species can change their colours rapidly, under irritation 

 or excitement. 



The body in the Cephalopoda is symmetrical, and is enclosed 

 in an integument which may be regarded as a modification of 

 the mantle of the other Mottusca. 

 Ordinarily there is a tolerably dis- 

 tinct separation of the body into 

 an anterior cephalic portion (pro- 

 somd), and a posterior portion, 

 enveloped in the mantle, and 

 containing the viscera (metasomd}. 

 The head is very distinct, bearing 

 a pair of large globular eyes, and 

 having the mouth in its centre. 

 The mouth is surrounded by a 

 circle of eight, ten, or more, long 

 muscular processes, or "arms" (fig. 

 224), which are generally provided 

 with rows of stalked or sessile 

 suckers. Each sucker, or "ace- 

 tabulum," 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 ap- 

 plied to any surface, the contrac- 

 tion of the radiating muscular Fig. 

 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 enlarged and club-shaped. In the 



224. Cephalopoda. 



Cuttle-fishes. 



