]\Ietazoa Obelia. 



227 



FIG. 117. TYPICAL MEDUSOID. (Hincks.) 



will serve also for that of the alimentary zooid in Obelia or 

 Hydr actinia. 



Turning now to the reproductive zooids, we are at once 

 brought face to face with the phenomenon of alternation of 

 generations, which we found to be so important a feature in 

 the life-history of plants. It is strange, however, that, whilst 

 it is the higher plants 

 that exhibit this pheno- 

 menon best, it is, on the 

 other hand, most per- 

 fectly developed in the 

 lower animals. The re- 

 productive zooid in 

 Obelia consists of a 

 short branch over which, 

 however, the perisarc is 

 continuous. The modi- 

 fied zooid within the 

 perisarc gives off buds, 

 consisting of protrusions 

 of both endoderm and 

 ectoderm. The bud in- 

 creases in size, and, at 

 the same time, takes on 

 a bell-shaped form with 

 a projecting tongue pen- 

 dant from the concavity. 

 We need not follow all 

 the stages in its development, but rather describe briefly 

 the completed product. The bud when ripe drops off and 

 swims away a free, independent organism, a gonophore or 

 medusoid. These medusoids were long looked on as distinct 

 animals, before their development from the fixed colony 

 was clearly made out. 



A medusoid, as already stated, is bell-shaped, the con- 

 cavo-convex body being known as the disc, or necto-calyx 



Q2 



