192 



THE ANIMAL KINGDOM 



p(z,z,ding 

 polyps 



polyps 



-Reproductive 

 polyps 



-Fishing 

 polyp 



Fig. 10.9 Fig. 10.10 



Figure 10.9. Pliysalia, the Portuguese man-of-war. (Courtesy New York Zoological 



Society.) 



Figure 10.10. A cluster of polyps from Pliysalia, showing the various modifications. 



(After Hyman.) 



as widely as possible over the prey. The edges of adjacent mouths meet 

 and enclose the prey completely. Then digestive juices are regurgitated 

 and the prey is disintegrated and swallowed. 



Of the several thousand species of hydrozoans none is of economic 

 importance. Usually the medusae are too small to be a nuisance to swim- 

 mers. A few kinds of polyps secrete limestone around the colonies and 

 thus contribute slightly to the building of coral reefs. 



67. Class Scyphozoa 



The medusa, which may be as much as a meter in diameter, is the 

 dominant stage in the class Scyphozoa. A common genus is Aurelia (Fig. 

 10.11), abundant in Atlantic and Pacific waters. Xematocysts of many of 

 the larger forms can penetrate the hinnan skin and produce intense pain. 



The mesoglea of scyphozoan medusae contains numerous scattered 

 ameboid cells of unknown fimction and distinct fibers that stiffen the 

 jelly-like matrix. The stomach is subdivided into a central chamber and 

 four gastric pouches, each containing internal endodermal tentacles 



