SUBKINGDOM PROTOZOA. 



21 



the marine Foraminifera nor among the Radiolaria. In the 

 fresh-water Foraminifera and Heliozoa it is, however, the 

 usual method in genera both with and without shells. 

 Where the -shell is thin it may be divided during the process, 

 but where it is thicker the protoplasm divides within it, one 

 of the new individuals retaining the old shell, while the other 

 wanders forth and constructs a new house for itself. This is 

 the case, for instance, in Arcella, in which the wandering indi- 



FlG. 9. Division of Am&ba (after SCHULZE). 



vidual protrudes from the mouth of the parent shell until it 

 forms its new shell, only separating when this is accom- 

 plished. 



Colonies, produced by repeated divisions and the imper- 

 fect separation of the forms so produced, are occasionally 

 formed, but they are simply aggregations of similar individ- 

 uals, no differentiation or iudividualizatiou of the colony as a 

 whole occurring. Among the fresh-water Rhizopods this is 

 the case with Microgromia, a shelled form, numerous individ- 



