FRESH-WATER POLYP AND SOME ALLIES 275 



most, grow into the adult form. The fixed stage is com- 

 parable with the hydroid generation of Hydrozoa. 



Aurelia is but slightly more dense than the sea water 

 itself. According to one authority there is less than one 

 eighth of 1 per cent of protein material present. Thus 

 over 99 per cent is water. The small amount of protein 

 material in the body of Aurelia makes it very unlikely 

 that many animals depend on them for food. After a 

 storm in summer great numbers of aureliae may be found 

 on the shore. In a few hours only scattered films are left on 

 the sand to show where the animals lay. The destruction 

 of many aureliae is to be expected because of their general 

 helplessness, but annihilation of the species is prevented by 

 the enormous number of young produced. 



Aurelia and its giant relative Cya'nea, which sometimes 

 grows to a diameter of eight feet, might threaten to fill the 

 ocean with their bodies, were it not for the fact that neither 

 lives over the winter. The young start their development 

 in the autumn and complete it in the spring. It is a self- 

 evident fact, in this instance at least, that death is a benefit, 

 not only to all other forms of sea life, but even to the race 

 of Aurelia itself. The adults swallow everything that can 

 be taken into the mouth, but probably their most abundant 

 food is small pelagic Crustacea similar to Cyclops (Fig. 92). 



The class of which Aurelia flavidula is a representative is 

 called Scyphozo'a (Gr. skyphos, "cup"; zoa, "animals"). 



The Sea Anemone. The best-known sea anemone of the 

 North Atlantic coast is Metrid'ium marginatum (Figs. 145, 

 146). The picture of the tide pool shows sea anemones 

 in various attitudes. Metridium has the power of moving 

 by creeping along on its base, but it seldom changes its 

 place of attachment, even in the coldest weather. 



The form of Metridium is nearly cylindrical. The oral 

 end of the column-shaped body, or polyp, expands into a 



