FRESH-WATER POLYP AND SOME ALLIES 273 



nonsexual 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 

 Hijdra and many thousands of species of hydroids. The life 

 history described for 

 Bougainvillia does not 

 apply to all species 

 of hydroids, but it is as 

 characteristic as any. 

 Hydrozoa are small 

 animals occurring sin- 

 gly 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 ectoderm and 

 theendoderm. In some 

 members of the class 

 the two layers are sep- 

 arated by a jelly-like mass secreted by the cells. The body 

 is radially symmetrical. Tentacles with nettling capsules in 

 their ectodermal cells are the organs of offense and defense. 



Larger Jellyfishes. Although the name "jellyfish " is some- 

 times applied to the medusa? of the class Hydrozoa, it is 

 more commonly given to the larger, saucer-shaped, or bell- 

 shaped, animals that swim at or near the surface in har- 

 bors and bays of all oceans. The species most frequently 

 found from Massachusetts southward is Aure'lia flavid'ula 

 (Fig. 144). This jellyfish reaches the diameter of fifteen 

 inches. 



Fig. 144. Photograph of a jellyfish. 

 (Reduced) 



