CEPHALOPODA (SQUID AND CUTTLEFISH). 



The Cephalopods derive their name from the fact that 

 the circle of tentacles or arms around the mouth (i.e., on 

 the head) was compared to the foot of other molluscs. Later 

 investigations show that these tentacles represent but a part 

 of the foot, the siphon also belonging to the same category. 

 These same arms, which are either eight or ten in number, 

 bear sucking organs by means of which these animals hold 

 fast their prey. In only the pearly nautilus are the arms 

 lacking, and here they are replaced by about a hundred 

 smaller organs. 



The head, which is separated from the body by a distinct 

 neck, bears a pair of eyes simple in the nautilus, but al- 

 most as complex as those of man in the other forms. In 

 these more highly developed eyes there is retina, lens, iris, 

 cornea, and cavities resembling those occupied by the 

 aqueous and vitreous humors. Yet the resemblances are 

 superficial; the structures are in reality totally different. 



The mantle is connected with the body in the region of 

 the so-called back. Below, it encloses a good-sized mantle- 

 cavity, open in front. It is very muscular, and the open- 

 ing about the neck can be closed at will, so that the only 

 connection between the mantle-chamber and the outside 

 world is through the tube of the siphon. If one of these 

 animals fill its mantle with water, close the neck opening, 

 and then force out the water by contracting the mantle, 

 the water will stream from the siphon in a strong jet, which 



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