CfELENTERATA. 179 



in the center of the concave side (Fig. 50). Around the extreme 

 margin is a fringe of tentacles. The bell can contract in such a way 

 as to force out the contained water and thus swim along slowly 

 through the water. There is still only the single cavity lined with 

 entoderm (gastro-vascular) , but it is more complex and branched 

 than in the polyp stage. All the parts are arranged with the most 

 precise radial symmetry. (Fig. 50.) 



When mature the medusa produces eggs or sperm or both, and 

 after fertilization the embryos develop, much as described in Section 

 114. They are free-swimming fora while and then settle down and be- 

 come attached. As this embryo develops, it forms not a medusa like 

 its parent, but a polyp like its grandparent. The medusa is a deli- 

 cate and fragile organism, and probably its individual life is quite 

 short. They are in this respect similar to some of the insects that 

 live onry a few days after their metamorphosis into the adult stage 

 just long enough to reproduce satisfactorily. 



194. Alternation of Generation. The process that has just been 

 described polyps reproducing non-sexually by budding and forming 

 medusae, which reproduce sexually and form polyps is known as 

 alternation of generation. The polyp stage enables the descendants 

 of one egg to become very thick over a small territory, by budding. 

 The free medusa, scattering its eggs, enables the species to take 

 possession of new territory. While the medusa dies after a period 

 of reproducing, the colony of polyps does not, unless overcome by a 

 disaster of some kind. 



While the alternation of generation is common in the Hydrozoa, it 

 is not universal. There are polyps that have no bell stage and 

 there are bells that have no polyp stage. * ' 



195. The Large or True Jelly-fishes (Scyphozoa) : A Library 

 Exercise. Examine the descriptions and figures in a number of 

 advanced texts and compare with the conditions in the smaller 

 jelly-fishes or medusae of the Hydrozoa. 



What are the difference as to the development and position of 

 the tentacles? Differences in the radial canals of the gastro-vascular 

 cavity? Is there a fixed stage? How does it differ from the tubular 

 polyps of the Hydrozoa? What is the nature of the non-sexual 

 reproduction? Is this an alternation of generation? 



196. The Sea Anemones. These are barrel-shaped organisms that 

 grow singly and attached to the rocks in shallow waters near shore. 

 They have great numbers of tentacles, and often present vivid 

 colors. They sometimes grow to great size, reaching one to one 



