322 



Descriptive Zoology. 



individual they produce sperms. The gonads set free 

 their contents in the water, and the eggs are fertilized by 

 the sperms. After fertilization, the egg develops first into 

 a simple hydralike polyp, and later, by budding, into a 

 branched hydroid like the form 

 that produced the medusa. 



Alternation of Generations. - 

 Thus we see that these hydroids 

 have two forms, and neither one is 

 complete. The hydroid form can 

 spread as a colony, but it can form 

 no new colony. But by means of 

 the medusae, which are free-swim- 

 ming, it forms new colonies, which 

 may be at a distance from the 

 original colony. This peculiar 

 process of development, hydroids 

 giving off medusae, and medusae, 

 in turn, producing eggs which 

 develop into hydroids, is known 

 as "alternation of generations." 

 It is found in some other lower ani- 

 mals, and among the Arthropods. 



Other Forms of Hydrozoa. The 

 great majority of Hydrozoa have 

 an alternation of generations as 

 described. But there are others 

 in which there is only a medusa 

 form, no polyp form appearing, 

 the eggs produced by the medusa 

 developing directly into a medusa form. In others the 

 medusa buds are produced, but are not set free. Whil 

 remaining attached, they produce and set free the 



FIG. 183. PORTUGUESE 

 MAN-OF-WAR. 



