HYDRA 



307 



blastostyles, escape through the distal pores, and develop to sexual 

 maturity as free-swimming individuals. The sexes of these are 

 separate; some produce eggs, and others, spermatozoa, which are 

 discharged into the water at maturity and unite to form zygotes. 

 The zygote develops into the free-swimming, ciliated planula stage 

 which soon attaches and develops into a polyp from which a new 

 colony arises. After producing a generation of medusae, this colony 



Fig. 107. — Physalia, the Portuguese man-of-war, a floating colonial coelenterate. 

 (From Hegner, College Zoology, The Macmillan Company.) 



disintegrates, and after producing germ cells, the medusae die. This 

 process, involving alternation of generation, is called metagenesis. 



Order TracJiylina. — This order includes two suborders of hydro- 

 medusae which come from the egg directly with no polyp stage. 

 Trachynema, Campanella, and Liriope are generic examples. 



Order Hydrocorallina. — This group resembles the corals by produc- 

 ing strong calcareous skeletons. They have extensive, branched hy- 



