THE PHYLA COELENTERATA AND CTENOPHORA 



195 



gastric pouches. Gametes are shed first into the pouches and then to the 

 outside through the mouth. 



In other respects the medusa o£ this class resembles that of the 

 Hydrozoa. The scyphozoan polyp is an inconspicuous part of the life 

 cycle. Asexual reproduction by frustule formation (Fig. 10.13, A) is 

 common. 



Medusae form by the direct transformation of the polyp head, rather 

 than by lateral budding as in the Hydrozoa. In some species the entire 

 polyp transforms into a single medusa. In others a series of medusae 

 may be produced. Successive medusae may overlap in development, so 

 that new medusae begin to form beneath older ones that have not yet 

 broken free. The result is a pile of partially formed medusae resembling 

 a stack of plates. This stage, which is shown by Aurelia (Fig. 10.13, B), 

 is called a strobila, and the process is called strobilization. 



Most of the 200 species of scyphozoans are similar and adhere to 

 the simple jellyfish plan. They are of little interest to man except as 

 nuisances to swimmers. 



68. Class Anthozoa 



In the third class, the Anthozoa, medusae are lacking and the polyps 

 become sexually mature. The polyps are usually short and stout (Fig. 

 10.14) with a large mouth and numerous internal partitions. The meso- 



omadeuni 



Figure 10.14. Metridium. Diagrammatic view of half a polyp. Several internal struc- 

 tures have been omitted. 



