242 SPH^:RELLA CHAP. 



forwards, and the forward movement is accompanied by a 

 rotation of the organism upon its longer axis. 



Careful watching shows that the outline of a swimming 

 Sphaerella does not change, so that there is evidently 

 no protrusion of pseudopods, and at first the cause of 

 the movement appears rather mysterious. Sooner or later, 

 however, the little creature is sure to come to rest, and there 

 can then be seen projecting from the pointed end two exces- 

 sively delicate colourless threads (Fig. 68, A, fl\ each about 

 half as long again as the organism itself : these resemble the 

 cilia on the epithelial cells lining the frog's mouth (p. 109), 

 except that they are few in number, and do not vibrate 

 rhythmically; they are therefore usually distinguished as 

 flagella. In a Sphaerella which has come to rest these 

 can often be seen gentty waving from side to side : when 

 this slow movement is exchanged for a rapid one the whole 

 organism is propelled through the water, the flagella acting 

 like a pair of extremely fine and flexible fins or paddles, 

 Thus the movement of Sphaerella is not amoeboid, i.e., 

 produced by the protrusion and withdrawal of pseudpods, 

 but is ciliary, i.e., due to the rapid vibration of cilia or 

 flagella. 



By staining and other tests it is shown that Sphaerella, 

 like Amoeba, consists of protoplasm, and that the flagella 

 are simply filamentous processes of the protoplasm. 



The green colour of the organism is due to the presence of 

 a special pigment called chlorophyll, the substance to which 

 the colour of leaves is due. That this is something quite 

 distinct from the protoplasm may be seen by treatment 

 with alcohol, which simply kills and coagulates the proto- 

 plasm, but completely dissolves out the chlorophyll, pro- 

 ducing a clear green solution. The solution, although green 

 by transmitted light, is red under a strong reflected light, 



