2/2 MOLLUSCA. 



attaining a length of an inch or more. Fifty-two species of 

 Tentaculites are enumerated by M. Barrande, commencing in 

 the Lower Silurian and ranging into the Devonian. The genus 

 is essentially Silurian, and examples of some species often 

 occur in myriads through a considerable thickness of strata. 



CHAPTER XXIV. 

 CLASS CEPHALOPODA. 



CLASS IV. CEPHALOPODA. The members of the Cephalo- 

 poda are denned 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 ' ' infundibulum " 

 or "funnel"}, through which the effete water of respiration is 

 expelled. 



The Cephalopoda, comprising the Cuttle-fishes, Squids, Pearly 

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

 classes of the Mollusca. They are all marine and carnivorous, 

 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 Nau- 

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

 this is extremely different in the two forms. 



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 toler- 

 ably distinct separation of the body into an anterior cephalic 

 portion (prosonta), 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 



