The Many-armed MoUusks 



acters. So are the circulatory and excretory systems. Though 

 isolated far from the other members, they are mollusks. 



Geologists have found a most thrilling history of the gigantic 

 prehistoric cephalopods written in the rocks. Regarding the 

 forms which had no hard parts, much is left to conjecture, but 

 the shell-bearing cephalopods are in evidence, remarkable in 

 variety, size and numbers, from the Upper Cambrian rocks, 

 through the Silurian, Devonian and Carboniferous eras, cul- 

 minating in the Mesozoic time, and dwindling in the Cenozoic. 



Silurian rocks yield fossil "straight-horns" fifteen feet long. 

 The gigantic Ammonites, with coiled and chambered shells four 

 feet in diameter, mark the maximum point of development 

 reached by the ancient tribes of Cephalopods. In the eras fol- 

 lowing the Cretaceous, which saw this wonderful tribe decline, 

 the mollusks are distinctly modern in size and genera. 



A brief account of these fossil cephalopods will be found in 

 any text-book of geology. 



Cephalopod mollusks are divided into tv/o orders, based upon 

 the number of gills and presence or absence of an external shell. 

 Subdivision into sub-orders is based upon the number of arms 

 that surround the head. A simple key will set forth these groups. 



KEY TO THE ORDERS AND SUB-ORDERS OF CEPHALOPODS 



A. Shell internal or absent ; arms, eight to ten ; gills, two. 



Order Dl BRANCH 1 ATA 

 B. Arms eight, all alike ; suckers fleshy ; shell absent. 



Sub-order Octopoda 



Eight-armed Cephalopods 

 The Argonauts 

 The Devil-fishes 

 BB. Arms ten, two elongated; suckers with horny rims; 

 shell internal. 



Sub-order Decapoda 



Ten-armed Cephalopods 

 The Spirula 

 The Cuttles 

 The Squids 

 AA. Shell external ; arms, more than ten; gills, four. 



Order TETRABRANCHIATA, 

 The Chambered Nautilus 



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