PHYLUM PROTOZOA. FLAGELLATES 



43 



genus Euglena; these differ from one another 

 in size, shape, behavior, and structural de- 

 tails. 



Morphology 



Euglena viridis (Fig. 18) is 0.1 mm. or 

 less in length, blunt at the anterior end, and 



pointed at the posterior end. Figure 18 pre- 

 sents the structural features of this species. 

 The peripheral layer of cytoplasm is a thin 

 elastic membrane, the pellicle. This pellicle 

 has parallel, spiral thickenings that give it 

 a striated appearance. It is rigid enough to 

 maintain the shape of the body, but suf- 

 ficiently flexible to allow euglenoid move- 



Cell mou^h 

 Cell gullet 



Flageltum 



Reservoir 

 Pellicle 



OJI 



Nucleus 

 Endosome 



•■Chloroplast 



Paramylum body 



Figure 18. Euglena viridis. Diagram of a stained specimen showing structure. 



ments. Near the anterior end is a funnel- 

 shaped depression, the cell mouth 

 (cytostome), that leads into the cell gullet 

 ( cytopharynx ) . The euglena does not eat 

 solid food as these terms might imply. The 

 cytopharynx is enlarged at the base to form 

 a vesicle called the reservoir, adjacent to 

 which is located a contractile vacuole, which 

 discharges its contents into the reservoir and 

 out through the cytopharynx. 



Near the anterior end of the body is an 

 orange-red eye spot which is part of a light- 

 sensitive organelle and probably serves in 

 orienting the euglena to light. A flagellum, 

 which arises from two axial filaments within 

 the body, extends out of the cytostome. The 

 electron microscope shows that the flagellum 

 consists of a core of two axial filaments sur- 

 rounded by a sheath of protoplasm. Near 

 the center of the euglena is an oval or spheri- 

 cal nucleus containing a central body, the 

 endosome. The function of the endosome is 

 controversial. Suspended in the cytoplasm 

 are also a number of green bodies, the 



chromatophores, which are known as chloro- 

 plasts. This green color is due to the pres- 

 ence of chlorophyl. In Euglena viridis the 

 chloroplasts are slender and radiate from a 

 central point. In each chloroplast of some 

 species of Euglena there is a pyrenoid, which 

 is probably a center for the formation of a 

 starchlike substance called paramylum. Para- 

 mylum bodies may also be free in the cyto- 

 plasm in the form of disks, rods, and links. 

 Paramylum is produced by photosynthesis 

 and represents reserve food material. 



Physiology 



Nutrition 



Euglenas obtain their nutriment largely 

 by photosynthesis, a process that takes place 

 within the chloroplasts; however, it has not 

 been proved that any species of Euglena can 

 grow in light without a trace of organic 

 material such as a peptone. In the dark the 

 euglenas can live on organic compounds that 

 are dissolved in water; under these condi- 



