HYDROZOA 



237 



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

 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 III. Scyphozoa.— The Scyphozoa arc the com- 

 mon jellyfiLshes. They are usually larger than the medusa^ 

 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 oi tour. 

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



