PROTOZOA 



down to the surface and slowly recover; the movement affects 

 the cilia successively in a definite direction in waves, and pro- 

 duces, like that of flagella, either locomotion of the cell or 

 currents in the medium. We can best realise their action by 

 recalling the waves of bending and recovery of the cornstalks in 

 a wind-swept field ; if now the haulms of the corn executed these 

 movements of themselves, they would determine in the air above 

 a breeze-like motion in the direction of the waves (Pig. 5).^ 

 Such cilia are not -infrequent on those cells of even the Highest 

 Animals that, like a mosaic, cover free surfaces ("epithelium cells "). 

 In ourselves such cells line, for instance, the windpipe. One 

 group of the Protozoa, the " Oiliata," are, as their name implies, 

 ciliated cells pure and simple. 



The motions of cilia and of flagella are probably also due to 

 changes of surface tension — alternately on one side and the other 



Fig. 5. — Motion of a row of cilia, in profile. (From Verworn.) 



in the cilium, but passing round in circular succession in the 

 flagellum,^ gi'ving rise to a conical rotation like that of a weighted 

 string that is whirled round the head. This motion is, however, 

 strongest at the thicker basal part, which assumes a spiral form 

 like a corkscrew of few turns, while the thin lash at the tip may 

 seem even to be quietly extended like the point of the corkscrew. 

 If the tip of the flagellum adhere, as it sometimes does, to any 

 object, the motions induce a jerking motion, which in this case is 

 reciprocating, not rotatory. When the organism is free, the 

 flagellum is usually in advance, and the cell follows, rotating at 

 the same time round its longitudinal axis ; such an anterior 

 flagellum, called a " tractellum," is the common form in Protista 

 that possess a single one (Figs. 29, 7, 8 ; 30, C). In the sperma- 

 tozoa of Higher Animals (and some Sporozoa) the fiagellum is 

 posterior, and is called a " pulsellum." 



The cilium or flagellum may often be traced a certain distance 

 into the substance of the cytoplasm to end in a dot of denser, 



' For discussions on the mechanism of ciliary action, see Sehafer, Anat. An:. 

 xxiv. 1904, p. 497, xxvi. 1905, p. 517 ; Schuberg, Arah. Protist. vi. 1905, p. 85. 

 ^ Like the line of most rapid growth in a ciroumnutatiug plant-stem. 



