III. EUGLENA 



Euglena is a common example of a very small group of uni- 

 cellular, animal-like organisms which possess chlorophyll — that 

 remarkable energy-trapping and food-synthesizing agent which, as 

 has been noted, is characteristic of green plants. The presence of 

 chlorophyll is considered by some authorities to be a fundamental 

 and decisive feature and they, therefore, regard Euglena as a plant 

 and place it in close relationship to the unicellular Algae, such as 

 Pleurococcus. Other authorities, notwithstanding the plant-like 

 nutrition, regard Euglena and similar forms as Protozoa because 

 their general structure is very similar to undeniably unicellular 

 animals. In other words, they regard Euglena as an animal which 

 in some way has acquired a plant-like type of nutrition. For the 

 present, Euglena may be considered as a border-line, or transitional 

 form showing a close relationship to the Protozoa in its general 

 structure and to the green plants in its nutrition. 



Thus it is evident that in certain of the lower forms of life, the 

 characteristics, which serve unmistakably to differentiate higher 

 plants and animals, disappear, so that it becomes impossible, when 

 considering these border-line forms, to say definitely that they are 

 either plant or animal. They partake of the characters of both 

 kinds of organisms. 



A. Structure of Euglena 



Several species of Euglena are known, all of which are micro- 

 scopic in size. Structurally they show a somewhat higher type of 

 organization than does the Amoeba. The outer surface of the 

 body is covered by a delicate, striated cell wall which is formed 

 as a secretion by the underlying ectoplasm. An active Euglena 

 exhibits peculiar squirming, or euglenoid movements, during 

 which the shape of the cell varies greatly, ranging from nearly 

 spherical to a typical cigar-shape. These changes in shape are 

 possibly due to the fact that the cell wall, or pellicle, is very thin 

 and, therefore, does not possess sufficient rigidity to hold the rather 



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