242 SPH^RELLA CHAP. 



end is always directed forwards, and the forward move- 

 ment is accompanied by a rotation of the organism upon 

 its longer axis. 



Careful watching shows that the outline of a swim- 

 ming 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 excessively delicate colourless 

 threads (Figs. 68, A, ft), 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 gently 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 with- 

 drawal of pseudopods, 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 sub- 

 stance 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 protoplasm, but completely dissolves out 

 the chlorophyll, producing a clear green solution. The 

 solution, although green by transmitted light, is red 



