In fact the cephalopods and some of the social insects may be 

 said to form the two culminating points of invertebrate develop- 

 ment. The insects are interesting chiefly for the complexity of 

 their instincts and social life, while the cephalopods are remark- 

 able for the perfection of their physical structure. They have 

 had an unusually long geological history, their remains being 

 found in rocks of every period since the Cambrian, when they 

 were probably masters of the sea, exceeding in size and activity 

 all other living animals. Again in the Cretaceous they were rep- 

 resented by a wonderful variety of large species, although by 

 this time the sea and land were dominated by vertebrate animals 

 of a much higher type. Today the more ancient division of the 

 cephalopods is represented by but a single living genus, the 

 Pearly Nautilus. This form is well known from the external 

 shell which envelops the body and which is susceptible of high 

 polish. The Pearly Nautilus lives in the Pacific Ocean in the 

 vicinity of Japan and the Philippines, but is decreasing in num- 

 bers and will probably become extinct in the comparatively 

 near future. 



Another cephalopod frequently confused with Nautilus but 

 not in any way closely related to it is the so-called Paper Nauti- 

 lus. This form has a thin shell bearing a general resemblance 

 to that of Nautilus, but is carried by the female only and is used 

 as a brood pouch for the care of the young. The scientific name 

 of this form is Argonauta, a term which avoids confusion with 

 Nautilus and refers to the animal's habit of sailing with the aid 

 of its light shell along the surface of the water. 



The cephalopods are divided into two groups, the dibranchs 

 and the tetrabranchs, according as they have two gills or four. 

 Nautilus is the only living tetrabranch. The dibranchs are in 

 turn divided into octopods and decapods, according as they have 

 eight arms or ten. Octopus has always been a synonym for all 

 that is grasping, little distinction being popularly made between 

 the forms with eight arms and those with ten. The arms in 

 question are really tentacles springing in a circle around the mouth 



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