MOVEMENTS OF AQUATIC MICRO-ORGANISMS 299 



The example just mentioned was not the starting-point of 

 my inquiries, which had been carried on during some years 

 previously. 1 I bring it forward merely as a particularly clear 

 case of the behaviour of minute, green, aquatic organisms when 

 acted upon by light of varying intensity. 



Any observer whose attention has been drawn to the 

 phenomena will be likely to put to himself such questions as 

 these : 



By what means are these living organisms enabled to rise 

 and fall in the water? 



What causes or enables them to arrange themselves in a 

 network ? 



If, as is probably the case, the organisms derive some benefit 

 from their peculiar movements and mutual arrangements, what 

 is the nature of the benefit ? 



These and like questions can only be answered satisfactorily 

 by experiments so conducted that the influential conditions can 

 be modified one by one. Euglctia viridis, a unicellular organism, 

 which often abounds in stagnant waters rich in organic matter, 

 has been found particularly suitable for the investigation. 



Under ordinary conditions, Euglena is a small free-swimming 

 organism, spindle-shaped, with an obliquely truncated fore end 

 and a pointed hinder end. The cell-protoplasm contains 

 chlorophyll-bodies which may be scattered uniformly except for 

 a clear space at the fore end or they may radiate from the centre, 

 leaving both ends free. Grains of a substance (paramylum) 

 resembling starch but not coloured blue by iodine may also 

 be found anywhere in the cell ; they are generally more 

 numerous in the neighbourhood of the chlorophyll-bodies. A 

 nucleus is usually seen near the hinder end of the cell. There 

 is a mouth on the under side of the fore end ; this leads by 

 a narrow pharynx to a large internal cavity. A pulsating 

 vacuole is present and there is a conspicuous red eye-spot on 

 the dorsal side of the pharynx. Locomotion is effected by a 

 single long flagellum, which — being at the anterior end — carries 

 the body forward by striking the water obliquely. The 

 organism revolves on its longitudinal axis as it travels, taking 

 a well-marked spiral path. Euglena, like all green plants, is 



1 See Wager, "On the Effect of Gravity upon the Movements and Aggre- 

 gation of Euglena viridis, Ehrb. and other Micro-organisms " {Phil. Trans. B, 

 vol. 201, 191 1). 



