HYDROZOA. 



41 



concavity of the bell or " umbrella " ; four radial cauals pass from Case 3. 

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

 of the umbrella ; from the same margin are suspended four pairs of 

 tentacles, each tentacle being provided at its base with an eye-spot. 

 The opening of the umbrella is partly closed by a narrow circular 

 band, the veil or velum, extending 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 relaxation of the 

 umbrella. 



The little Medusa is simply an extremely modified polyp, specially 



Fig. 2.' 



Colony of Boiigainvillea fruticosa, natural size, attached to the underside of a 

 piece of floating timber (After AUinan.) 



adapted for a free-swimming existence. If a Hydra were shortened 

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

 Medusa would result. The Medusa has nervous and muscular 

 apparatus and sense organs in correspondence with its active 

 existence, these being absent in the fixed polyps. 



A colony of Bougainvillea, then, is made up of individuals of two 

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

 digest food for the colony, and generative polyps, whose function it 

 is to form and carry the eggs. Here we have division of labour, and 



' From " Encyclopaedia Britannica." 



