96 ELEMENTARY ZOOLOGY 



In case of many polyps all or some of the new indi- 

 viduals which arise by budding do not become polyps, but 

 develop into medusae or jellyfish, which separate from the 

 fixed polyp and swim off through the water. These 

 medusae or jellyfish produce sperm-cells and egg-cells. 

 The sperm-cells fertilize the egg-cells and a new indi- 

 vidual develops from each fertilized egg. This new indi- 

 vidual is at first an active free-swimming larva called a 

 planula, which does not resemble either a medusa or polyp. 

 After a while it settles down, becomes fixed and develops 

 into a polyp. Thus a polyp may produce a medusa or 

 jellyfish which, however, produces not a new jellyfish, but 

 a polyp. This is called an alter nation _gf generations . 

 and is not an uncommon phenomenon among the lower 

 animals, j It results from such an alternation of genera- 

 tions that a single species of animal may have two distinct 

 forms. This having two different forms is called diino}'- 

 pJiism. Sometimes, indeed, a species may appear in 

 more than two different forms ; such a condition is called 

 polymorphj^tn . 



Not all medusae or jellyfish are produced by polyp indi- 

 viduals, nor do jellyfish always produce polyps and not 

 jellyfishes. There are some jellyfishes (we might call 

 them the true jellyfishes) which always have the jellyfish 

 form, producing new jellyfishes either by budding or by 

 eggs, and there are some polyps which always have the 

 true polyp form, producing new individuals, either by 

 budding or by eggs, always of polyp form and never of 

 jellyfish form. That is, some species of Ccelenterata 

 exist only in polyp form, some species exist only in jelly- 

 fish form, while some species (those having an alternation 

 of generations) exist in both polyp and jellyfish form, ] 

 these two forms appearing as alternate generations. 

 / Classification. The branch Ccelenterata is divided f 

 ^ four classes: (i) the Hydrozoa, including the fresh- 



