56 THE HALL OF SHELLS. 
“The immense shoals 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 known 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 disappoimtment. 
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 
