COELENTEEATA 41 



of Hydra. The food obtained by the nutritive zooids is 

 circulated in the general gastric cavity by the action of flagella 

 developed by the cells of the endoderm. 



Medusoid 

 Gonophore 

 Gonotheca 



Blastostyle 



Tentacle 



Hypostome 



Zooid 



Hydrotheca 



Hydrocaulus 



Perisarc 

 Coenosarc 



Hydrorhiza 

 FIG. 17. The hydroid stage of Obelia geniculaia. 



Eeproduction takes place by the development of special 

 outgrowths from the axillae of the branches of the stem. 

 The outgrowth involves the ectoderm, the endoderm, and the 



