CEPHALOPODA. 



273 



of eight, ten, or more, long muscular processes, or "arms " 



(fig. 246), which are generally provided with rows of suckers. 



In the Octopod Cuttle-fishes there are only eight arms, and 



these are all nearly alike. In the 



Decapod Cuttle-fishes there are 



ten arms, but two of these called 



" tentacles " are much longer 



than the others, and bear suckers 



only at their extremities, which 



are enlarged and club-shaped. 



In the Pearly Nautilus the arms 



are numerous, and are devoid of 



suckers. 



The parts of the Cephalopoda 

 which may be preserved in a fossil 

 condition, and which thus inte- 

 rest the palaeontologist, are the 

 mandibles, the ink-bag, and the 

 skeleton, whether this be internal 

 or external. 



The mandibles are contained 

 within the mouth or " buccal 

 cavity" of the animal, and have 

 the form of powerful jaws, work- 

 ing vertically like the beak of a Fig . 246 ._ Ce phaiopoda. 

 bird. They are horny or calcare- Atiantica, one of the Cuttle-fishes 



j i ii (after Woodward). 



ous, and in shape closely resem- 

 ble the beak of a parrot, with this difference, that the largest 

 of the two mandibles is placed inferiorly. Mandibles of this 

 nature are present in both the Cuttle-fishes and the Pearly 

 Nautilus, and they doubtless existed in all the extinct forms. 

 They not uncommonly occur as fossils, but they do not appear 

 to have been observed out of the Jurassic and Cretaceous 

 Rocks. They are commonly called " Rhyncholites/' and genera 

 such as Rhynchoteuthis have been founded upon them (fig. 

 247). 



The ink-bag is a special gland possessed by the Cuttle-fishes, 

 for the purpose of secreting an inky fluid, which the animal can 

 discharge into the water, so as to enable it to escape when 

 menaced or pursued. The secretion of the ink-bag consists of 

 finely-divided particles of carbon suspended in fluid, and it is 

 extremely indestructible. The ink-bag, with its contained se- 

 cretion, is not uncommonly found in the fossil condition ; but 

 it has only been observed in strata of Secondary age. In the 

 Tetrabranchiate Cephalopods, in which there is an external 



s 



