136 VORTICELLA AND ZOOTHAMNIUM LESS, xn 



contractile vacuole (c. vac) are found in both the bell-shaped 

 and the globular zooids. 



It is to these globular, mouthless zooids that the functions 

 of reproducing the whole colony and of ensuring dispersal 

 are assigned. They become detached, swim about freely 

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

 (p 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. 



