REPRODUCTION AND DEVELOPMENT 261 



medusa sheds the mature eggs or sperms into the water where a 

 chance meeting with the germ cell of the opposite sex results in 

 fertilization. The fertilized egg by cell divisions becomes a two- 

 layered flat PLANULA, which then develops into the attached branch- 

 ing asexual hydroid. Thus the cycle is complete. The description 

 here given applies in detail to the hydroid species Obelia, but the 

 essential features apply to all other species. 



The Scyphozoa. The free swimming medusa, sexual genera- 

 tion of the hydroid, very closely resembles a jelly fish. The jelly 

 fish constitute the second class of Coelenterata, the class Scyphozoa. 

 In general the jelly fish are larger than the sexual generation of the 

 Hydrozoa, have a somewhat more complicated set of radial canals, 

 and more clearly defined nerve elements. The sexes are separate and 

 in most species the fertilized eggs develop directly into free swim- 

 ming forms; a few species exhibit a sort of metagenesis and some 

 have stalks that serve as hold-fasts. 



The Actinozoa. The members of the class Actinozoa are at- 

 tached marine Coelenterata, the endodermal cavity being divided 

 by a number of partitions into radial compartments (Fig. 35). The 

 sea anemone is a micmber of this class, but perhaps the most familiar 

 form is the coral, of which there is a wide variety of species. Corals 

 are colonial forms, composing large masses. True metagenesis does 

 not occur among the Actinozoa. The attached animal is sexual, the 

 sexes being separate. The gonads are located in the cavity and the 

 sex cells are expelled into the surrounding water. After fertilization 

 the eggs develop into planuL^ which in turn become the attached 

 animals. 



Viewing the Coelenterata as a group it is seen that the hydra is 

 the most simple; hydroids resemble the hydra in many respects but 

 by specialized buds give rise to a free swimming sexual generation, 

 which in turn resembles the class Scyphozoa. In the Actinozoa 

 there is a return to the attached form; the gonads, however, are 

 internally placed as in the free swimming Scyphozoa. 



