IV 

 THE CEPHALOPODA 



THE present-day Cephalopoda (cuttle-fish, squid, octopus 

 and nautilus) are the most highly organized of Mollusca. 

 They agree with gastropods in having a rasping tongue, 

 but the crawling foot is replaced by a series of powerful 

 arms around the mouth, and the mantle and body-wall 

 become muscular swimming organs. While the crawling 

 life of gastropods has made them asymmetrical, and only 

 a few have acquired a superficial symmetry through 

 taking to a swimming life, nearly all cephalopods retain 

 the perfect bilateral symmetry, while as active voracious 

 animals their sense-organs have become highly de- 

 veloped. Only a few extinct forms are asymmetric, and 

 these may well have been crawlers exclusively, while 

 the rest of those known as fossil may, like the modern 

 members of the class, have been at least capable of 

 swimming and sometimes swimmers exclusively. The 

 habitat of cephalopods is more restricted than that of 

 gastropods, being exclusively marine. 



Most of the modern cephalopods, such as the cuttle- 

 fish and squid, show no sign of a shell externally, 

 though it is to be found in a modified condition buried in 

 the interior of the body ; some, as the octopus, have 



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