HYDROZOA. 
41 
concavity of the bell or “umbrella ” ; four radial canals pass from Case 
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. 1 
Colony of Bougainvillea fruticosa, natural size, attached to the underside of a 
piece of floating timber. (After Allman.) 
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 
1 From “ Encyclopaedia Britannica.’ 
