OTHER CCELEXTERATA 277 



Alternation of Generations in Obelia. — We are now in a position 

 to consider the unusual type of life cycle that is represented by 

 such a hydroid as Obelia. The colony with its three types of 

 individuals arises from a single parent polyp by the asexual process 

 of budding. The medusae, which arise as buds detached from the 

 colony, are individuals, specialized for swimming, which upon 

 reaching maturity develop either testes or ovaries. The ova and 

 spermatozoa are shed into the water where they meet in fertiliza- 

 tion. After producing its crop of medusae, the colony dies; and 

 after producing its ova or spermatozoa, the medusa dies. Thus 

 one may say that there are two generations which alternate: the 

 hydroid generation, reproducing asexually; and the medusa gen- 

 eration, reproducing sexually. To such life cycles in animals, 

 the term alternation of generations, or metagenesis, has been 

 applied. 



Not all hydroid polyps and hydro-medusce exhibit such a cycle. 

 There are polyps like the hydra, with no medusa stage and no 

 real colony formation; and medusa?, like Gonionemus, in which 

 there is no well-developed hydroid colony. In some medusae, as, 

 for example, Liriope, the fertilized egg develops so directly that 

 if the species were taken by itself one would never suspect the 

 existence of an elaborate colonial stage in some of its near relatives. 

 There are also hydroids such as Clava which, Uke hydra, have 

 merely reproductive organs and produce no medusa^. To appre- 

 ciate the many interesting comparisons that can be made, it is 

 necessary to make a more extended study than is possible here. 

 One can, however, get some understanding of the relationships by 

 considering how the life cycle of Hydra could be modified to 

 make it like that of Obeha; or how the life cycle of a medusa like 

 Liriope could be modified to make it like that of Obelia. 



Other Hydrozoa. — Related to the hydroids and the hydro- 

 medusae, as jellyfishes Uke Gonionemus are called, are the hydroid 

 corals, or HydrocoraUinoe. These have a massive skeleton of 

 carbonate of lime formed by the ectoderm, and thus when dried 

 resemble somewhat the skeletons of the true corals. The extinct 

 fonns represented by the fossils called Graptolites probably belong 

 to the Hydrozoa. IVIost interesting of all are the Siphonophora, 

 of which the Physalia, or ''Portuguese man-of-war" (Fig. 134), 

 found floating in the Gulf Stream and occasionally blown upon 

 the coast, is a familiar example. Its structure cannot be con- 



