THE SEA-ANEMONE AND SOME ALLIES 267 



outside. The embryo swims about for awhile and then set- 

 tles to some fixed or floating object, attaches itself, and soon 

 another colony of the hydroid (hydra-like) stage is developed 

 by budding. The life-history of Bougainvillia illustrates 

 alternation of generations. The fixed, colonial, non-sexual, 

 hydroid stage alternates with the free-swimming, single, 

 sexual, medusa stage, and together they complete the life- 

 cycle. 



Definition of Hydrozoa (Gr. Hydra, the fabulous monster; 

 zoa, animals). The class Hydrozo'a includes the genus Hydra 

 and many thousands of species of hydroids. The life-history 

 described for Bougainvillia by no means applies to all spe- 

 cies of hydroids, but it is as characteristic as any. Hydrozoa 

 are small animals occuring singly and in colonies. There is 

 but one cavity in the body, and that is continuous with the 

 mouth-opening. There are but two layers of cells, the ecto- 

 derm and the endoderm. In some members of the class the 

 two layers are separated by a jelly-like mass secreted by the 

 cells. The body is radially symmetrical. Tentacles with net- 

 tling-capsules in their ectodermal cells are the organs of 

 offense and defense. 



The Jellyfish. Although the name " jellyfish " is sometimes 

 applied to the medusae of the class Hydrozoa, it is more com- 

 monly given to the larger, saucer-shaped or bell-shaped animals 

 that swim at or near the surface in harbors and bays of all 

 lands. The species most frequently found from Massachusetts 

 southward is Aure'lia flavid'ula (Fig. 134). This jellyfish 

 reaches the diameter of fifteen inches. 



In various details of structure Aurelia resembles the 

 medusa of certain hydroids, and also the sea-anemones, the 

 coral polyp, and the fresh-water polyp. The mouth opens at 

 the center of the under surface into a large cavity which 

 branches freely into many tubes running to the circumference, 

 where they join the circular canal. Between the ectoderm 



