TYPE CCELENTERA. 93 



their maturity in the medusae or medusoid buds except in the 

 HydrarisB, in which this stage is entirely wanting, and in cer- 

 tain Hydrocorallinse, in which it has disappeared, and may 

 likewise first become differentiated in the medusa. In many 

 forms, however, in which an alternation of generations occurs 

 they arise in the polyp, sometimes at a point far distant from 

 where the medusa-buds will arise, and reach these structures 

 only after, it may be, a rather extensive series of wanderings. 

 Thus, to take an extreme case, in Evdendrium the ova arise 

 in the ectoderm of the main stem of the pinnately branching 

 colony a short distance below the terminal hydranth ; as new 

 branches are formed in this same region the young ova 

 migrate into them, passing through the supporting layer and 

 wandering among the eudoderm cells. Later on when the 

 gonopolyps arise on the lateral branches the ova wander into 

 them, still in the endoderm, and finally when the medusa- 

 buds develop on the gonopolyps the ova continue their endo- 

 dermic course into them and eventually, again passing through 

 the mesogloea, take up their final position in the medusoid 

 ectoderm. Gradations between such an extensive migration 

 and cases in which none occurs are to be found, and it may be 

 stated as a general rule that the more the medusoid buds 

 depart from the medusa form the greater is the migration 

 undergone by the reproductive cells. 



As a rule the Hydromedusse are of separate sexes, the sepa- 

 ration affecting the entire colonies — or, to put it slightly dif- 

 ferently, the medusae are always unisexual, and a polyp colony 

 when it occurs gives rise to medusae or medusoid buds ail of 

 the same sex. The Hydrarise, however, are exceptions to the 

 rule, being hermaphroditic. 



A blastula results from the segmentation of the ovum, and 

 this is converted into either a sterrula by immigration or a 

 diblastula (i.e. a hollow two-layered organism without mouth 

 or tentacles) by delamination (see p. 55). If a sterrula be 

 formed, it assumes the diblastula condition by a hollowing out 

 of the central mass, and after swimming about for some time 

 in this condition, if a polyp is to be formed, it settles down 

 upon some foreign body, a mouth breaks through and ten- 

 tacles appear, producing the first hydranth. 



