IT FLAGELLA 25 



the forward movement is accompanied by a rotation of the 

 organism upon its longer axis. 



Careful watching shows that the outline of a swimming 

 Haematococcus 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. 3, A,y7), each about 

 half as long again as the animalcule itself : these are called 

 flagella or sometimes cilia}- In a Haematococcus 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 

 Haematococcus is not amceboid, i.e., produced by the pro- 

 trusion and withdrawal of pseudopods, but is ciliary, i.e., 

 due to the rapid vibration of cilia or flagella. 



The flagella are still more clearly seen by adding a drop 

 of iodine solution to the water : this immediately kills and 

 stains the organism, and the flagella are seen to take on a 

 distinct yellow tint. By this and other tests it is shown that 

 Haematococcus, like Amoeba, consists of protoplasm, and 

 that the flagella are simply filamentous processes of the 

 protoplasm. 



It was mentioned above that in swimming the pointed end 



1 The word cilium is sometimes used as a general term to include 

 any delicate vibratile process of protoplasm : often, however, it is used 

 in a restricted sense for a rhythmically vibrating thread, of which each 

 cell bears a considerable number (see Fig. 8, E, and Fig. 21) ; a flagel- 

 lum is a cilium having a whip-lash-like movement, and each cell 

 bearing only a limited number one or two, or occasionally as many 

 as four. 



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