224 EVENINGS AT THE MICROSCOPE. 



themselves. It is, however, an optical illusion, depending 

 on the nature of ciliary movement, which therefore I must 

 first endeavour to explain to you. 



Cilia are organs which play a very important part as 

 instruments of locomotion, as well as of other functions, 

 in all the lower forms of animals, and in the early stages 

 of some of the higher forms. They are also found cha- 

 racterising the lowest form of vegetable life, giving to them 

 the means of spontaneous locomotion, which renders them 

 liable to be mistaken for animals. They consist of pro- 

 longations of the fleshy tissue into long and very delicate 

 hairs, which are endowed with a special faculty of motion. 

 This consists of a bending down in a given direction to a 

 certain extent of flexure, followed by a rapid resuming of 

 the perpendicular ; which is, however, immediately suc- 

 ceeded by like bendings and straightenings in alternate 

 gradation. The simplest condition of this movement is 

 that in which a single cilium only exists, by whose succes- 

 sive lash-like beats upon the surrounding water the ani- 

 mal is rowed along like a boat through the sea. But far 

 more commonly cilia are arranged in rows, or in many 

 series of rows, in which ease the bending and straightening 

 of the individual cilia do not occur otherwise than in strict 

 and orderly relation to each other. For instance, one 

 cilium in a given row begins to bend, the one next to it 

 then begins, then the third, then the fourth, and so on, all 

 precisely in the same direction, all in precisely the same 

 time, all with precisely the same force, and all to precisely 

 the same extent. It follows, that before the first has com- 

 pleted its beat and resumed the erect position, three or 

 four others are in various degrees of flexion, regularly 

 graduated ; and that if the eye could look laterally at such 

 a row of cilia suddenly arrested and fixed as they were, 

 we should see their tips tracing a wavy line instead of a 

 straight one. Moreover, since the bending of any cilium 

 brings its tip nearer to its successor than it was before, 



