THE PHYLUM PROTOZOA 



155 



fresh waters. They tend to occur as "blooms," becoming extremely 

 abundant for a short time and then disappearing. Although most species 

 are harmless and form an important source of food for other organisms, 

 a few produce deadly toxins. Most spectacvdar are the small reddish 

 forms that color the water when they become abundant and produce 

 "red tides." Such water is lethal to fish, killing them rapidly as they 

 enter the poisoned region. The dead animals decay and enrich the sup- 

 ply of nutrients so that a red tide, once started, tends to be self-per- 

 petuating until ^vater currents or storms break it up. 



Many dinoflagellates are parasitic. Although the adult forms seldom 

 resemble dinoflagellates they can be identified by their young, which 

 have the typical grooves and flagella. 



Phytomonads. An order of more plantlike flagellates are the phy- 

 tomonads (order Phytomonadina). They are all autotrophic, but have 

 visual organelles and swim about in a most animal-like fashion. Within 

 the group is a series of colonial species, ranging in complexity from 

 the one-celled Clihiinydomonas to the highly integrated, spherical 

 colony, J'olvox (Fig. 8.6). Volvox represents the peak of protozoan 

 colony formation. Although each individual of the colony feeds for 

 itself, they are not all alike. The pigment spots are largest in the cells 

 at one pole of the sphere, which is always the anterior pole in loco- 

 motion, and decrease steadily in size around to the posterior pole. Re- 

 production is limited to the equatorial and posterior cells. All cells are 



Gon.iura 





Volvox: 



lorina 



Figure 8.6. Examples of the order Phytomonadina showing solitary form {Chlamy- 

 domonas), simple colonies (Goniitm, Eudorina). and a complex colony {Volvox). Colonial 

 forms are embedded in a matrix of transparent jelly. 



