394 



GENERAL ZOOLOGY 



adult hydra. In many of the Hydrozoa there is metagenesis 

 in the life cycle, i.e., an alternation of polyp with medusoid 

 generations. The polyp, developing from fertilized egg, repro- 

 duces asexually by budding the medusae, which are male and 

 female. 



Fig. 223. — Physalia, the Portuguese man-of-war, a pelagic colonial hydrozoan. 

 cr, crest; p, polyp; pn, pneumatophore. (From Parker and Haswell, Textbook of 

 Zoology, copyright, The Macmillan Company. By permission.) 



Regeneration is marked in Hydra. Grafting of parts can also 

 be experimentally accomplished without difficulty. 

 CLASS I. HYDROZOA. Radially symmetrical, sessile, polyp 

 forms, and free-swimming sexual medusae; both sometimes 

 occurring in the life history of a single species. Without going 

 into the intricacies of classification, these different conditions 

 may be illustrated by the following: 



