340 THE LIVING ANIMALS OF THE WORLD 



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OCTOPODS 

 A tlut-tfettta Witt Auaralia* ifteiei 



particular modification of the locomorfve 

 organ, or foot, serves, on the one hand, to 

 readily distinguish the first or most highly 

 organised group from the second or central 

 class; while the third or lowest one is as 

 clearly separated from the second and first by 

 the character of the shell. The first and most 

 highly developed section includes such species 

 as the Octopus, the Cuttle-fish, the Squid, 

 and the several varieties of Nautiluses; to 

 the second or central group are referred 

 all the marine and terrestrial Slugs and 

 Snails with their innumerable modifications; 

 while the third and lowest group comprises 

 all the double-shelled or bivalve forms, such 

 as Oysters and Mussels. 



In the OCTOPUS and its allies the creeping- 

 base, or foot, is prolonged round its margin 

 into eight or more arm-like extensions. The 

 anterior of these in the earlier phases of their 

 development grow round and enclose the 

 mouth, which consequently opens upon the 

 centre of the locomotive surface. The arms 

 of the octopus and other forms are in ni"-t 

 instances provided throughout their length 

 with complex sucking-disks, wherewith thrir 

 owners can seize and retain their prey or 

 walk with case and celerity over the smoothest 

 perpendicular or overhanging rock-surfaces. 

 The octopus, as the type of its class, possesses 



which is so richly supplied with muscular 

 tissues as to constitute an effective creeping- 

 base. As a locomotive organ this muscular 

 area is usually known as the " foot." 



Of living molluscs some 50,000 distinct 

 species have been recorded. The great 

 majority of these organisms are, as is well 

 known, marine. A very considerable number, 

 however, are inhabitants of fresh-water ; while 

 a yet smaller proportion, like the Slugs and 

 Snails and their allies, are especially adapted 

 for a terrestrial existence. Kxcepting two 

 relatively small and inconspicuous groups, the 

 great natural division or sub-kingdom of 

 Molluscs is separated by systematic zoologists 

 ; nto three main sections or classes. The 



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A TASMANIAN SQUID, OR CUTTLE-FISH 



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