CORALS, SEA-ANEMONES AND JELLY-FISHES 65 



it settles down, becomes fixed, and develops into a polyp. 

 Thus a polyp may produce a medusa or jelly-fish which, how- 

 ever, produces not a new jelly-fish, but a polyp. This is called 

 an alternation of generations, and is not an uncommon phenom- 

 enon among the lower animals. It results from such an al- 

 ternation of generations that a single species of animal may 

 have two distinct forms. This having two different forms is 



FIG. 1 6. A jelly-fish, or medusa, Gonionema vertens, eating two small 

 fishes. (From specimen from Atlantic Coast.) 



called dimorphism. Sometimes, indeed, a species may appear 

 in more than two different forms; such a condition is called 

 polymorphism. 



The Portugese man-of-war, common in tropical seas, is a 

 representative of another group of this class. It appears 

 as a delicate bladder-like float, brilliant blue or orange in 

 color, usually about six inches long, and bearing on its upper 



