PHYLUM CCELENTERATA 



165 



two kinds of individuals at the ends of the branches: 

 feeding hydranths (h)' and gonangia (g) . Obelia is therefore 

 an example of polymorphism, a condition in which a single 

 species appears in more than one form. 



Po/yp 



Medusa 



FIG. OS. Indicating the fundamental structural similarity between gastrula, 

 polyp, and medusa. The polyp is a gastrula with tentacles; the medusa is like 

 a polyp that has grown upward around the tentacles in the region of x. a, 

 archenteron, or primitive digestive cavity; ra, mouth. 



It has been said that all ccelenterates are modified gas- 

 trulas, and this will hold true for both hydranths and 

 gonangia. In a hydranth a circlet of tentacles has grown 

 about the gastrula 's mouth, which is simply the original 

 opening into the archenteron (Fig. 64, G). The gonangium 



