MOLLUSCA : CEPHALOPODA. 275 



presence of four gills, and, though abundantly represented in 

 past time, has no other living representative than the Pearly 

 Nautilus alone. 



CHAPTER LI. 

 DIVISIONS OF THE CEPHALOPODA. 



ORDEE I. DIBEANCHIATA. The members of this order of the 

 Cephalapoda are characterised as being swimming animals, 

 almost invariably naked, with never more than eight or ten 

 arms, which are always provided with suckers. There are 

 two branchiae, which are furnished with branchial hearts ; an 

 ink-sac is always present ; the funnel is a complete tube, 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 weapons, being 

 excessively tenacious in their hold, and being sometimes pro- 

 vided with a sharp claw in the centre of each sucker. They 

 are mostly nocturnal or crepuscular animals, and they some- 

 times attain to a great size. They may be divided into two sec- 

 tions Octopoda and Decapoda, according as they have simply 

 eight arms, or eight arms and two additional ' tentacles.' 



SECTION A. OCTOPODA. The Cephalopods comprised in this 

 section are distinguished by the possession of not more than 

 eight arms, which are provided with sessile suckers. The 

 shell is internal and rudimentary ; in one instance only (the 

 Argonaut), external. 



This section comprises the two families of the Argonautidce, 

 and the Octopodidce. In the former of these there is only the 

 single genus Argonauta (the Paper Sailor, or the Paper Nau- 

 tilus), of which the female and male differ greatly from one 

 another. The female Argonaut (Jig. 84) is protected by a thin 

 single-chambored shell, in form symmetrical and involuted, 

 which is secreted by the webbed extremities of the dorsal arms, 

 but is not attached in any way to the body of the animal. It 

 sits in its shell with the funnel turned toward the keel, and 

 the webbed arms applied to the shell. The male Argonaut is 

 much smaller than the female (about an inch in length), and 

 is not protected by any shell. The third left arm is deve- 

 loped in a cyst, and ultimately becomes a ' hectocotylus,' and 

 is deposited by the male in the pallial chamber of the female. 



In the Octopodidce (or Poulpes), there are eight arms, all 

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