56 THE HALL OP SHELLS. 



" The immense slioals of these microscopic 

 animals not only help at night to produce what 

 is called the 'phosphorescent sea,' but by the 

 light of day give their color to the waves. 

 Thus the variety kno^n as ' whale food ' red- 

 den the sea for miles, and when whalers per- 

 ceive their ruddy hue upon the waves they 

 realize they have reached the ' pastures of the 

 whales.' " 



By this time their walk had ended and 

 Tom announced his " think tank full." When 

 the party again visited the spot where they 

 had left the Portuguese man-of-war, the chil- 

 dren could find no trace of their singular ac- 

 quaintance, and but for wiser companions 

 would have returned home in disappointment. 

 They were shown, however, several filmy ob- 

 jects, scarcely more than an inch across, flat, 

 flabby, and semitransparent. These they 

 learned were the air sacs of their jellyfish, and 

 were told that even a medusa, weighing several 

 pounds and with many yards of tentacles, when 

 he dropped his "mortal coil" left only sea 

 water and air sacs ; these latter Tom presented 

 to Undine as " the skeleton of the Portuguese 

 man-of-war." 



The colony of Hydroids growing about a 

 truncated harp shell in our engraving gives a 



