284 ESSENTIALS OF ZOOLOGY 



process is paramylum, a granular substance much like starch. Grains 

 of this substance may be observed throughout the endoplasm of these 

 Euglenae when living in favorable conditions. It is not likely that all 

 three of these fundamental types of nutrition are found in any one 

 species of Euglena, but all are represented in closely related species 

 of these flagellates. 



Respiration and Excretion 



Respiration is carried on through the general surface of the cell 

 membrane. There may be some utilization of the carbon dioxide pro- 

 duced in the metabolic activity by the process of photosynthesis in 

 forms where it exists. Likewise, some of the excess oxygen produced by 

 photosynthesis may be used in metabolism. Water and waste products 

 collect in the several small contractile vacuoles which empty into the 

 reservoir, a permanent vesicle communicating with the exterior. 



Reproduction and Life Cycle 



Binary longitudinal fission is the common means of reproduction. 

 This division occurs only in the motile state (or active phase) in 

 some species, in a quiet but not encysted condition in other species, 

 and in a few others, fission occurs only while encysted (encysted 

 phase). E. viridis may divide by longitudinal binary fission in either 

 the motile or encysted condition. According to some authors the 

 original flagellum is retained by one-half, while a new flagellum is 

 developed by the other, but there is also some rather authentic work 

 which shows that the old flagellum entirely disappears during divi- 

 sion, and a new one is developed in each daughter cell. During ad- 

 verse conditions, such as drought or increased chemical concentration, 

 Euglena becomes encysted. In this condition it becomes spherical 

 in shape, nonmotile, and secretes a thick gelatinous envelope about 

 itself. During the encysted phase, division takes place. There may 

 be a single division or there may be several. Upon the return of 

 normal, favorable conditions these cells emerge from the cyst and 

 assume the active phase. Some observers have reported as many as 

 thirty-two young flagellated individuals coming from a single cyst. 

 On rare occasions two individual Euglenae come together side by 

 side and fuse permanently into a single cell. This is somewhat similar 

 to the zygote formation in sexual reproduction. 



