Phylum Protozoa 



355 



In their reactions to light, the Euglena demonstrate positive photo- 

 taxis; that is, they move toward the light source. In general, Euglena 

 avoid extremes of light and dark, preferring areas with a medium 

 illumination. Ordinarily Euglena move through water in a spiral 

 due to the action of the flagellum. Also they are capable of distinctive 

 crawling motions which are called euglenoid motions. 



Fig. 112. — Some colonial Phytomastigina. A, Volvox sp.; entire colony, show- 

 ing daughter colonies; B, Volvox sp., section of surface of colony, showing proto- 

 plasmic bridges; C, Volvox sp., bundle of microgametes; D, Volvox sp., an individual 

 microgamete; E, Volvox sp., an individual macrogamete; F, Volvox sp., zygote; 

 G, Pandorina sp.; H, Platydorina sp.; I, Gonium sp. 



Volvox. — Among many of the species of the subclass Phytomasti- 

 gina, individuals join together to form colonies. While these may seem 

 very similar to a multicellular form, they demonstrate no true tissue 

 differentiation. Each vegetative cell is precisely like every other one; 

 however, there may be some differentiation of sex cells in a form such 

 as Volvox (Fig. 112,^ -F). 



