POLYZOA. 



245 



verge towards the central mouth. By this wonderful 

 provision two ends are obtained ; the particles of 

 water are incessantly renewed for the purpose of 

 respiration, and by the same means every minute 

 substance, animal or vegetable, that happens to be 

 swimming in the neighbourhood, is dragged by the 

 ceaseless whirlpool into the midst of the ciliated 

 tentacles, and thus brought to the mouth, where 

 such as are fit for prey are seized and swallowed. 



On the very Flustra we have been describing, 

 there often exists an example of a parasitic pro- 

 duction, belonging to the same class, the structure 

 of which is still more marvellous. 



The Sea Moss (Bowerbankia densa) forms beneath 

 the microscope an object of matchless beauty. This 

 little parasitic Polyzoon (Fig. 185) consists of in- 

 numerable polype-like creatures, each inhabiting 

 an extremely delicate transparent tube ; clusters of 

 these glassy cells arise from a creeping stem, common 



FlG. 185. BOWERBAXKIA. 



to the whole group. Examined with a microscope, 

 the tubes in which these minute animals live are 



