34 ANIMAL LIFE 



hairs derive their force not from a muscle connecting 

 them with the body, but from an inherent flexibility, 

 conferring on them the power of beating more rapidly 

 in one direction than in the reverse ; and as they 

 maintain this vibratory movement with almost perfect 

 constancy, rarely stopping to rest, still more rarely 

 reversing their action, the body of the animalcule 

 is carried through the water in a direction opposite 

 to that of the more forceful stroke. If we imagine 

 the body provided with a membrane which bends 

 rapidly into a curve, straightens backwards, and 

 again bends forwards, we should have in this membrane 

 an organ acting not unlike the tail of a prawn or the 

 effective bend of a fish-tail. Now let us imagine the 

 membrane to be composed of hairs which flex and 

 straighten of their own accord ; then, if close set and 

 synchronous in their stroke, their effectiveness in 

 propelling the animal would hardly be less than that 

 of a continuous membrane, and as the animalcules are 

 excessively minute aquatic creatures, very small hairs, 

 exerting a comparatively weak force, are sufficient to 

 move them. 



These hairs, or ' cilia,' as they are called, from a 

 supposed likeness to eyelashes, arc not confined to 

 animalcules. They are found on the bodies of flat, 

 gliding worms of our streams and ponds and on the 

 foot of snails. They are the means whereby the 

 minute young of all sorts of marine animals, which 

 crawl or walk when older, swim about. The sea- 

 jellies, eggs of worms and snails, hatch out as micro- 



