MOLLUSCA: CEPHALOPODA. 299 



CHAPTER L. 

 CEPHALOPODA. 



CLASS IV. CEPHALOPODA. The members of this class are 

 defined by the possession of eight or more arms placed in a 

 circle round the mouth ; the body is enclosed in a muscular 

 mantle-sac, and there are two or four plume-like gills within 

 the mantle. There is an anterior tubular orifice (the " infun- 

 dibulum " or "funnel"), through which the effete water of re- 

 spiration is expelled. The flexure of the intestine is neural. 



The Cephalopoda, comprising the Cuttle-fishes, Squids, 

 Pearly Nautilus, &c., constitute the most highly organised of 

 the classes of the Molhtsca. They are all marine, and carnivo- 

 rous, and are possessed of considerable locomotive powers. At 

 the bottom of the sea they can walk about, head downwards, 

 by means of the arms which surround the mouth, and which 

 are usually provided with numerous suckers or " acetabula." 

 They are also enabled to swim, partly by means of lateral 

 expansions of the integument or fins (not always present), and 

 partly by means of the forcible expulsion of water through the 

 tubular " funnel," the reaction of which causes the animal to 

 move in the opposite direction. 



The majority of the living Cephalopods are naked, possess- 

 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 nume- 

 rous little sacs, 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 Mollusca. Ordinarily there is a 

 tolerably distinct separation of the body into an anterior 

 cephalic portion (prosoma), and a posterior portion, enveloped 

 in the mantle, and containing the viscera (metasoma). 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. 107), which are generally provided with rows of 

 suckers. Each sucker, or " acetabulum," consists of a cup- 



