THE CCELENTERATES 



207 



colony and swim away as jellyfish or medusae. The medusa? 

 produce eggs and spermatozoa. The eggs are fertilized by the 

 spermatozoa, and these fertilized eggs develop into hydroid 

 colonies. This rather complicated life history is described here 

 for the purpose of illustrating the phenomenon of alternation 

 of generations, also known as metagenesis. The asexual genera- 

 tion is represented by the budding colony; this produces the 

 medusae, which give rise to germ cells and thus constitute the 

 sexual generation. 



Jellyfish. — Some jellyfishes or medusae belong to the 

 class Hydrozoa along with Hydra and the hydroids just de- 



Fig. 123. — A jellyfish swimming. (From Jammes.) 



scribed, but most of them are placed in the class Scyphozoa. 

 The mesoglea (see p. 203) of these animals is very thick, giving 

 them a jellylike consistency. Medusae are for the most part 

 disk-shaped with a fringe of tentacles around the edge and oral 

 arms hanging down around the mouth (Fig. 123). They swim 

 slowly about in the sea by means of gentle undulations of the 



body. 



Sea Anemones. — Sea anemones are cylindrical animals with 

 a crown of tentacles often so beautifully colored as to resemble 



