871 



CHAPTER XLIV. 



DIBRAXCHIATE CEPHALOPODS. 



The Dibranchiate Cephalopods or " Cuttle-fishes " 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 branchice, which are furnished with branchial hearts ; 

 an ink-sac is always present ; the funnel is a complete tube ; a?id the 

 shell (when present) is internal, or, if external, is not chambered. 



The Cuttle-fishes (fig. 804) are rapacious and active animals, 

 swimming 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 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. 



The general anatomy of the Cuttle-fishes has already been briefly 

 discussed (see p. 821), and it only remains to allude shortly to cer- 

 tain points which possess a special palseontological interest. Under 

 certain exceptionally favourable circumstances the outline of the 

 body has been preserved in the fossil Dibranchiates, and in such 

 cases the horny hooks with which the suckers are occasionally fur- 

 nished may be recognisable, and it may even be possible to deter- 

 mine the number of the "arms." The mandibles of the Cuttle- 

 fishes differ from those of the Tetrabranchiates in not being calcified, 

 and these structures are therefore not preserved in the fossil condi- 

 tion. On the other hand, the Dibranchiates possess in the ink-sac 

 a structure which is quite capable of petrifaction, the carbonaceous 

 particles suspended in the " ink " being very indestructible. Hence 

 the fossilised ink-bag of the Dibranchiates is of tolerably frequent 

 occurrence. 



The skeleton in the Dibranchiates may be rudimentary or wholly 

 absent, as in the majority of the Octopods. In the female Argo- 

 naut, again, there is a delicate, involute, external shell, which is 



