104 OCTOPODS AND DECAPODS. 



reduces its specific gravity considerably below that of the sur- 

 rounding medium, and is able to mount almost without an 

 effort to the surface ; while, when there, the mere act of with- 

 drawing itself within the shell will instantly suffice to send it 

 again to the bottom. 



It is not necessary, therefore, to suppose that the siphon is in 

 any way concerned in these movements ; and in the absence of 

 proof, all theorizing based on the assumption that it is so is a 

 mere gratuitous exercise of ingenuity. By far the most proba- 

 ble function of the tube is, that it helps to maintain the vitality 

 of those portions of the shell which are furthest removed from 

 the body of the animal, and which, if deprived of all vital con- 

 nection with it, would speedily become fractured, and thus totally 

 destroy the hydrostatic balance of the Nautilus, and doom it for 

 ever after to remain at the sea-bottom. 



The two-gilled Cephalopods comprise the Octopods, distin- 

 guished by the possession of eight arms or feet, and the Deca- 

 pods, which boast the possession of an additional pair of those 

 organs. 



Let us glance at the ten-armed gentlemen first. In all these 

 animals two of the ten arms are greatly prolonged, and provided 

 at the extremity with a thick fleshy expansion covered with 

 suckers, and, in one family, with powerful hooked claws, for 

 holding on firmly to their slippery prey. The Common Cuttle- 

 fish may be taken as the type of this division of the Cephalopods, 

 which also includes the Calamaries or Squids, the Spirula, and 

 that engaging little fellow, the smallest of the Cephalopods, the 

 common Sepiola. In many of these animals we meet with the 

 rudiments of an internal shell, as in the " bone " of the Cuttle- 

 fish, and the "pen" of the common Calamary, while in the 

 Spirula there is a true chambered shell, though only partially en- 

 closed in the body. The Cephalopods of this section are fur- 

 nished along each side of the body with a powerful muscular fin, 

 which greatly aids them in their movements through the water ; 

 and, in some of the Calamaries, these lateral fins are so largely de- 

 veloped that the animals can project themselves above the surface 

 of the water, and, like the flying-fish, dart for a short distance 

 through the air. Immense numbers of these " Flying Squid '' 

 annually visit the shores of Newfoundland, where they form the 

 principal bait employed in the Cod-fishery. 



