96 



THE STORY OF LIFE IN THE SEAS. 



and warmer seas of the world the surface may 

 be covered with thousands of little creatures 

 which, when brought upon the deck, seem to be 

 little else than coloured bladders of air. The 

 scientific name of these animals is Physalia. 

 When placed in a glass of water, however, it 

 will be seen that, from the under side of the 

 bladder which floats freely on the water, 

 numerous delicate tentacles and Polyps hang 

 down. These creatures are kept at the surface 

 by an air-bladder float and no muscular energy 

 is required to sustain them in that position. 



Another Siphonophore called Velella has a 

 bladder of a more complicated character in the 



shape of a disc 

 with a semi- 

 circular or 

 triangular sail 

 on its upper 

 side. There 

 can be no 

 doubt of the 

 advantage of 



this float to the species. It not only enables 

 them to keep afloat without the expenditure 

 of muscular energy, but as the wind catches the 

 sail they are drifted along over great areas of 

 the ocean and thus distributed far and wide from 

 the spot on which they were hatched. Still 

 the float has undoubtedly its disadvantages, for 

 it exposes them to the danger of being blown 

 ashore by a steady wind and so perishing in 

 thousands. Agassiz says that on the coast of 

 Florida the beach is sometimes marked with 



FIG. 26. 

 The swim-bladder of Velella. 



