160 TOILERS IN THE SEA. 



colony, and enter upon an independent existence. 

 In the latter case they present such a contrast to the 

 original stock, both in form and habit of life, that it 

 seems difficult to believe that the new forms have 

 any relation whatever to the old, but that they are 

 rather new creatures, belonging to quite a different 

 tribe. It is only by tracing their relationship that 

 one is thoroughly convinced of the reality of the 

 metamorphosis. In most cases the free zooid, which 

 is the product of the sexual bud, assumes the form 

 known to marine zoologists as that of the Medusa. 

 For the benefit of those who are not marine zoolo- 

 gists we must explain what these Medusa-forms are 

 like (fig. 26). 



A small delicate crystal dome, a little more 

 than a hemisphere, floating with the flat side down- 

 wards, and propelled through the water by con- 

 traction of the sides, its outline so delicate, and its 

 substance so glassy, that it can be just distinguished 

 in the water. The lower flat surface covered like a 

 kettle-drum, which it resembles in miniature, with a 

 thin membrane, with a round hole or opening in the 

 centre. The margin of the flat surface apparently 

 notched, or beaded, with a circlet of small sacs, and 

 a fringe of long thin tentacles, which hang down 

 waving and floating in the water like threads, such 

 is the general external appearance of the Medusa. 

 The wall of this swimming-bell is so thin and filmy, 



