134 VORTICELLA AND ZOOTHAMNIUM 



are assigned. They become detached, swim about freely 

 for a time, then settle down, develop a stalk and mouth 

 (r 1 , F 2 ), and finally, by repeated fission, give rise to the 

 adult, tree-like colony. 



The Zoothamnium colony is thus dimorphic, bearing indi- 

 viduals of two kinds : nutritive zooids, which feed and add to 

 the colony by fission, but are unable to give rise to a new 

 colony, and reproductive zooids, which do not feed while 

 attached, but are capable, after a period of free existence, of 

 developing a mouth and stalk, and finally producing a new 

 colony. Dimorphism is a differentiation of the individuals 

 of a colony, just as the formation of axial fibre, gullet, con- 

 tractile vacuole, and cilia are cases of differentiation of the 

 protoplasm of a single cell. 



