i6 



ECONOMIC ZOOLOGY 



metrical and have a single gastrovascular cavity. Many of them 

 exhibit two stages the polyp or hydroid form and the medusa or 

 jelly-fish stage. They possess minute sting cells or nematocysts. 

 They are usually divided into three or four classes, as follows: 



1. Hydrozoa, Fig. 8, fresh water polyps (Hydra, for example); 

 hydroids (Obelia); small jelly-fish (Gonionemus); and a few stony 

 corals. 



2. Scyphozoa, Fig. 9, mostly large jelly-fish, sometimes as large 

 as a wash-tub. 



3. Anthozoa or Actinozoa, Fig. 10, flower-like sea anemones, corals, 

 etc. 



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A EC 



FIG. 9. Three Scyphozoa. Somewhat reduced. A, Tessera princeps. B, 

 Periphylla hyacinthina. C, Charybdea marsupialis. (From Hegner, College Zoology, 

 after Sedgwick and Haeckel.) 



4. Ctenophora, Fig. n, comb-jellies or sea walnuts, very transparent 

 jelly-like forms with eight bands of comb-like, radially arranged, 

 ciliated plates or locomotor organs. This class is often described 

 as a distinct phylum. 



Hydrozoa. Except for comparatively unimportant sales to colleges 

 and museums this class has practically no economic importance so 

 far as is known; possibly they may help destroy organic waste particles 

 in the water. 



Scyphozoa. The same may be said of this class, except that the 

 common jelly-fish or sea nettles are sometimes a pest along bathing 

 beaches on account of the painful effect upon the human skin of their 



