ii PHYLUM PROTOZOA 61 



each of which becomes a flagellula (Fig.. 44, E, F) provided with a 

 single flagellum. In some instances all the spores produced are 

 alike (E), and each encloses a small crystal (c.) : in other cases (F) 

 in the same species the spores are dimorphic, some being small 

 (microspores) others large (megaspores). Their development has not 

 been traced. 



Symbiosis. One most characteristic and remarkable feature 

 of the group has yet to be mentioned. In most species there occur 

 in the extra-capsular protoplasm minute yellow cells (Fig. 40, z.) 

 multiply by fission independently of the Radiolarian. It is 



FIG. 44. Collozoum inerme, A C, three forms of the entire colony, nat. size ; D, a small 

 colony showing the numerous central capsules (c. caps.) and extra-capsular protoplasm with 

 vacuoles(vac.) ; E, spores containing crystals (c.) ; F, mega- and microspore. (From Butschli's 

 Protozoa, after Hertwig and Brandt.) 



now known that these are unicellular plants belonging to the class 

 of Algse and to the species ZooxantJiella nutricola. This intimate 

 association of two organisms is called symbiosis : it is a mutually 

 beneficial partnership, the Radiolarian supplying the Alga with 

 carbon dioxide and nitrogenous waste matters, while the Alga gives 

 off oxygen and produces sugar and other food-stuffs, some of which 

 must make their way by diffusion into the protoplasm of the 

 Radiolarian. 



CLASS II. MYCETOZOA. 



1. EXAMPLE OF THE CLASS Didymium difforme. 



Didymium occurs as a whitish or yellow sheet of protoplasm (Fig. 45, G), 

 often several centimetres across, which crawls, like a gigantic Amreba, over 

 the surface of decaying leaves. It shows the characteristic streaming move- 

 ments of protoplasm and feeds by ingesting various organic bodies, notably the 

 Bacilli which always occur in great numbers in decaying substances. Numerous 

 nuclei are present. 



