HYDROZOA 237 



the colonial millepore corals; and the colonial campanularian and tubu- 

 larian hydroids. The colonial forms are all fixed and usually have a free 

 medusa stage with alternation of generations. 



Order 2. The Siphonophora are swimming colonies in which there is a 

 highly developed stock polymorphism in which certain individuals form 

 floats or swimming bells. The sexual generation is either a free medusa 

 or an attached medusoid bud. Physalia, the "Portuguese Man of War," 

 is a well-known example. 



FIG. 133. A hydroid colony. X i. 



528. Class HI. Scyphozoa. The Scyphozoa are the com- 

 mon jellyfishes. They are usually larger than the medusae 

 of the Hydrozoa, varying from four inches to a yard in diameter. 

 The bell is usually strongly convex and is made up of four anti- 

 meres though most of the organs occur in multiples of four. 

 The margin of the bell is lobed, with as many sense organs 



