422 THE ANIMALS AND MAN 



Among the coral reefs in the South Seas there lives an 

 enormous kind of sea anemone or polyp. Individuals 

 of this great polyp measure two feet across the disk when 

 fully expanded. In the interior, the stomach cavity, which 

 communicates freely with the outside by means of the large 

 mouth opening at the free end of the polyp, there may often 

 be found a small fish {Amphiprion percula). That this 

 fish is purposely in the gastral cavity of the polyp is proved 

 by the fact that when it is dislodged it invariably returns 

 to its singular lodging place. The fish is brightly colored, 

 being of a brilliant vermilion hue with three broad white 

 cross bands. The discoverer of this peculiar habit suggests 

 that there are mutual benefits to fish and polyp from this 

 habit. "The fish being conspicuous, is liable to attacks 

 which it escapes by a rapid retreat into the sea anemone. 

 Its enemies in hot pursuit blunder against the outspread 

 tentacles of the anemone and are at once narcotized by the 

 'thread cells' shot out in innumerable showers from the 

 tentacles, and afterwards drawn into the stomach of the 

 anemone and digested." 



Small fish of the genus Nomeus may often be found accom- 

 panying the beautiful Portuguese man-of-war (Physalia) 

 as it sails slowly about on the ocean's surface. These little 

 fish lurk underneath the float and among the various hanging 

 thread-like parts of the Physalia, which are provided with 

 stinging cells. The fish are protected from their enemies 

 by their proximity to these stinging threads. Similarly, 

 several kinds of medusae are known to harbor or to be 

 accompanied by the young, or small adult fishes of the 

 genera Caranx and Psenes. 



Hermit crabs live in the shells of molluscs, most of the 

 body of the crab being concealed within the shell, only the 

 head and grasping and walking legs protruding. In some 

 species of hermit crabs there is always to be found on the 

 shell near the opening a hydroid polyp. "This hydroid 



