87 



CHAPTEE XXVIII. 



DIBRANCHIATE CEPHALOPODS. 



The Dibraiicliiate Cephalopods or Cuttle-fishes are character- 

 ised as being swionming animals, almost invariahly naked, ivith 

 never more than eight or ten arms, ivhich are ahvays provided 

 'with suckers. There are two hranchioi, ivhich are furnislied 

 ivith hranchial hearts ; an inh-sac is aliuays present ; the fun- 

 net is a complete tiibe, and the shell is internal, or, if external, 

 is not chambered. 



The Cuttle-fishes are rapacious and active animals, swim- 

 ming freely by means of the jet of water expelled from the 

 funnel. The arms constitute powerful offensive weajDons, 

 being excessively tenacious in their hold, and being some- 

 times provided with a sharp claw in the centre of each 

 sucker. They are mostly nocturnal or crepuscular animals, 

 and they sometimes attain to a great size. They may be 

 divided into two sections, Octopoda and Decapoda, according 

 as they have simply eight arms, or eight arms and two addi- 

 tional " tentacles." 



The parts of a Dibranchiate Cephalopod which may be 

 preserved in a fossil condition are the mandibles, the ink- 

 sac, the shell (if such be present), and the internal skeleton. 

 The occurrence of the ink-sacs of Dibranchiate Cephalopods 

 in a fossil state has been already spoken of (p. 55), and 

 need not be further noticed here. An external shell is 

 jjresent only in the Argonaut amongst living Cuttle-fishes, 

 and similar structures are of rare occurrence as fossils in 

 some of tlie youngest portions of the earth's crust. The 



