772 



MICROSCOPIC FORMS OF ANIMAL LIFE 



tapering gradually from the base to the point. Their size is ex- 

 tremely variable, the largest that have been observed being about 

 -Jinth of an inch in length, and the smallest about i3^oth. When 

 in motion each filament appears to bend from its root to its point, 

 returning again to its original state, like the stalks of corn when 

 depressed by the wind ; and when a number are affected in 

 succession with this motion, the appearance of progressive waves 

 following one another is produced, as when a cornfield is agitated 

 by successive gusts. When the ciliary action is in full activity, 

 however, little can be distinguished save the whirl of particles in 

 the surrounding fluid ; but the back stroke may often be perceived, 

 when the forward stroke is made too quickly to be seen, and the 

 real direction of the movement is then opposite to the apparent. In 

 this back stroke, when made slowly enough, a sort of 'feathering' 

 action may be observed, the thin edge being made to cleave the 



FIG. 592. A, Kerona stlnrus: a, contractile vesicle; 6, 

 mouth ; r, c, animalcules swallowed by the Kerona, after 

 having themselves ingested particles of indigo. B, 

 Paramecium caudatum: a, a, contractile vesicles; 

 b, mouth. The dotted lines indicate currents. 



liquid which has been struck by the broad surface in the opposite 

 direction. It is only when the rate of movement has considerably 

 slackened that the shape and size of the cilia, and the manner in 

 which their stroke is made, can be clearly seen. Their action has 

 been observed to continue for many hours, or even days, after the 

 death of the body at large. As cilia are not confined to animal- 

 cules and zoophytes, but give motion to the zb'ospores of many 

 protophytes, and also clothe the free internal surfaces of the respi- 

 ratory and other passages in all the higher animals, including man 

 (our own experience thus assuring us that their action takes place, 

 not only without any exercise of will, but even without conscious- 

 ness), it is clear that to regard animalcules as possessing a ' voluntary ' 

 control over the action of their cilia is altogether unscientific. 



