134 CEPHALOPODA. 



cartilage, answering to the skull of the Vertebrate 

 animals. From this centre nerves pass to the 

 organs of sense, to the arms, the mantle, and the 

 various internal organs. 



Besides the rudimentary skull, there are found 

 other pieces of cartilage in different parts of the 

 body, as among the muscles at the base of the 

 funnelj and (in the Squids) two narrow plates for 

 the support of the side-fins. Thus we have the first 

 approach to the internal bony skeleton, which is so 

 characteristic of the Vertebrata. 



Many species, however, possess also a calcareous 

 shell. In the Nautilus and the Argonaut, this is 

 external and ample : but in our native species 

 which possess one, the shell is internal, being 

 enclosed within the substance of the mantle, fall- 

 ing out when the latter is slit open. In the 

 Squids, its form is that of a clear, flexible, horny 

 pen or feather ; but in the Cuttle it is an oval, 

 shelly plate, carrying on one of its surfaces a mul- 

 titude of very thin plates, arranged in layers, kept 

 apart by microscopically minute pillars. Thus, 

 though formed of stony substance, it is so light as 

 to float in water, imparting buoyancy to the animal. 



The form of the body in general is more or less 

 that of an oval sac (the mantle), from which the 

 head, with its staring eyes and its array of arms, 

 protrudes in front, as does also the funnel ; while 

 its sides are in some species dilated into broad 

 swimming-fins. 



One of the most curious circumstances in the 

 economy of the Cuttles, is the constantly changing 

 play of various colours which is observed on their 

 suxface. During life there are seen coloured spots, 

 which are continually altering their position, size, 



