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EVOLUTION OF ANIMALS 



Part V 



Fig. 31.13. The gray sea-slug feeds on sea anemones such as the one on the 

 left; another one at the upper right has its tentacles withdrawn. Sea-slugs are mol- 

 lusks that lose their shells in early life and commonly bear gill-like filaments 

 brightly colored, translucent and continually moved by the currents of water. 

 Length of slug 4 inches. (Photograph courtesy, Douglas P. Wilson, Marine Bio- 

 logical Laboratory, Plymouth, England.) 



unevenly so. The shell of a squid is a horny quill pen embedded in flesh; that 

 of a nautilus is a many-chambered dwelling. All of the tribe are marine. No 

 other mollusks approach them in travel, the drifting of the female paper 

 sailor, the darting of the squids and sepias (Figs. 31.15, 31.16). Clam and 

 snail shells have great beauty of color, but no other mollusks can display the 



