PHYLUM COELENTERATA 



135 



in the internal cavity. With the exception of reproductive proc- 

 esses, a single hydranth of Obelia will be found similar to an entire 

 hydra, to be studied soon. 



The reproductive cycle is both sexual and asexual, alternating 

 between the sexually produced polyp or hydroid generation and 

 the asexually produced sexual generation, the medusa or jellj'fish 

 form. The medusae arise as buds from the special individuals, 

 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. 



Fig. 60. — Diagram of a siphonophore colony (Physophorida) . A. Pneuniatophore ; 

 B, C, swimming bells ; D, protective zooid ; E, sporosac : F. G, dactylozooids ; H, 

 feeding polyps (gastrozooids) ; /, nettling cells. (From Van Cleave, Invertebrate 

 Zoology, published by McGraw-Hill Book Company, after Claus.) 



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 

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

 process, involving alternation of generation, is described as meta- 

 genesis in Chapter VIII. 



Obelia presents a very good example of metagenesis as represented 

 in animals. The medusae of this sort are spoken of as hydromedusae 

 to distinguish them from the scyphomedusae or jellyfishes of Class 

 Scyphozoa. 



