Chap. 21 THE PROTOZOANS 433 



of movements. Excess fluids and metabolic products collect in its enlarged 

 base from whence they are discharged from the body. The reddish, light sensi- 

 tive eyespot is a markedly animal characteristic. The whole body is enclosed 

 in a thin elastic cover or pellicle that adjusts itself easily to the organism's 

 squirming movements. 



Nutrition. The chloroplasts are vital organs, the centers of photosynthesis by 

 which the carbohydrate food is formed with the help of water, carbon dioxide, 

 and radiant energy from the sun. Free-living flagellates also absorb dissolved 

 nutrient materials from the water in which they live; in fact, in nutrient solu- 

 tions euglenas will live and multiply even in the dark after losing their chlo- 

 rophyll. Their stored paramylum is a food similar to the glycogen in the tissues 

 of multicellular animals. Chlorophyll-bearing flagellates are the constructive 

 organisms of their communities. In both fresh and salt water they are the great 

 carbohydrate producers. 



Pigments. Phytoflagellates may be yellow green, blue green, orange, and at 

 times some are red. The colors are due mainly to carotene and allied pigments 

 that cloak the chloroplasts that are then called chromoplasts. Like the related 

 green of chlorophyll, the pigments of carotene are generally sensitive to light. 

 Protozoans that contain chromoplasts usually have a reddish stigma or eyespot 

 similar to that of the euglenas. "Red snow" and pasture pools "colored by red 

 rain," common in midwestern United States, are usually due to dense popula- 

 tions of red euglenas {Euglena rubra) (Fig. 21.6). 



Colonies. Some phytoflagellate colonies contain but a few individuals, 4, 8, 

 16 and thereabout, held together in jelly; others such as Volvox contain thou- 

 sands of them (Fig. 21.7). Many colonies show distinct polarity or difference 

 between the ends; in Pleodorina and Volvox the individuals at the anterior 

 pole are sterile while those farther back produce new colonies asexually by 

 repeated cell divisions. Volvox and others reproduce sexually and asexually; 

 some cells enlarge and become eggs, others divide and produce flagellate sperm 

 cells. The fertilized egg secretes a shell in which it can remain for a long period, 

 through drought or winter. When favoring conditions return, the egg divides 

 and a young colony emerges. 



Dinoflagellates. Composing a large part of the microscopic surface fauna of 

 the sea, dinoflagellates include the luminescent noctilucas that float in coastal 

 waters, and the armorbearers that are typical plankton forms of both ocean 

 and inland waters (Fig. 21.8). Dinoflagellates usually bear two flagella, each 

 one originating in a groove of the body surface. Their bodies are clothed in 

 membranes, or in two shells or several plates. Thus they are armored and earn 

 their name, dino or terrible flagellates. Their nutrition is generally plant-like 

 but some have lost the chromoplasts, have become ameboid, and feed on small 

 organisms in typical ameboid fashion. Still others get their living as parasites 

 in the intestines of copepods and other small floaters of the sea. Larger inverte- 



