208 SYSTEMATIC ZOOLOGY. 



ning of the last century was directly due to the borings of 

 these forms. 



CLASS V. 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 the pearly nautilus 

 only 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 are retina, lens, iris, 

 cornea, and cavities resembling those occupied by the 

 aqueous and vitreous humors (see Vertebrates). 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 onlv 



CD i/ 



connection between the mantle-chamber and the outside 

 world is through the tube of the siphon. This is a tubular 

 structure on the ventral surface, in no way homologous 

 with the similarly named tubes of the Acephala. If one 

 of these animals fill its mantle with water, close the neck 

 opening, and then force out the water by contracting the 



