COELENTERATA 39 



hydroid stage, briefly described above and (2) the 

 medusa or jellyfish stage. As we have just seen, 

 this plant-like hydroid possesses reproductive 

 polyps; by a form of budding these polyps produce 

 minute jellyfish. In many species these break loose 

 from the colony and become free-swimming hydro- 

 medusae. Although very minute when liberated, 

 they may attain a size of an inch or so at maturity. 

 In some species they always remain very minute and 

 attached to the hydroid stalk. 



When the medusa is mature, it produces eggs 

 which soon develop into minute free-swimming or- 

 ganisms known as planuae. Soon, however, these 

 planulae settle down and attach themselves to some 

 object and grow, plant-like, into a hydroid. Thus 

 the cycle is completed. The animal has passed 

 through an alternation of two generations (1) the 

 hydroid stage which reproduces by plant-like bud- 

 ding (asexually) to form (2) the medusa stage which 

 in turn reproduces by eggs (sexually) to again form 

 the hydroid. 



These two forms, hydroid and medusa, are 

 totally different in appearance and one would never 

 suspect that they belonged to the same species. In 

 fact, early naturalists did not realize this and gave 

 different names to the two stages. 



Not all Hydrozoa pass through the same life 

 history. Some species have only a hydroid stage, 

 while others have only a medusa. One group 

 (Siphonophora) consists of large free-floating com- 

 munities. The Portuguese Man of War (Physalia 

 pelagica) is the best known example of this group, 

 (see page 59) 



Most of the hydroids are very small and are 

 easilv overlooked by the casual collector. The 



