HYDROZOA. 



■II 



concavity of the bell or " umbrella " ; four radial i-analK pass from C%m 3, 

 the central stomach to open into a circular canal round the margin 

 of the umbrella ; from the same mart'lu are suspended four [jairt of 

 tentacles, each tentacle beini; provided at its base with an eye-sjxjt. 

 The opening of the umljrelhi is i)artly closed by a narrow circular 

 baud, the veil or velum, extendiug in from the rim ; and, most 

 important of all, the eggs are situated in the walls of the manubrium. 

 The organism swims by alternate contraction and ndaxation of the 

 umbrella. 



The little ^ledusa is simply an extremely modified polyp, sjK'cially 



Fi.;. 2.' 



-^^: 



^^m^^-^ 



Colony of BougainvilJea fruticosa, natural size, nttached to the underside of a 

 piece of floating timber. (After AUman.) 



adapted for a free-swimming existence. If a Hi/(ha were shorteue<l 

 and drawn out laterally (Fig. 5), a shape somewhat like that of a 

 jMedusa would result. The ^Medusa has nervous and muscular 

 apparatus and sense organs in correspoudence with its a<-tive 

 existence, these being absent in tlie fixed polyps. 



A colony of Bouoamrillea, then, is uiade up of indivitluals of two 

 kinds, feeding or nutritive polyps, whose function it is to obtain aud 

 digest food for the colony, aud generative polyps, whose funetion it 

 is to form and carry the eggs. Here we have division of lal)our, and 



' From " Encycloi)a.Hlia Britannieu. 



