258 



EVENINGS AT THE MICKOSCOPB. 



rapid revolution on their axes. This was believed to 

 be the real fact by the earlier microscopists, though 

 they were utterly unable to conceive how such a move- 

 ment could consist with parts maintaining an organic 

 connexion between themselves. It is, however, an op- 

 tical illusion, depending on the nature of ciliary move- 

 ment, which therefore I must first endeavor to explain 

 to you. 



Cilia are organs which play a very important part 

 as instruments of locomotion, as well as of other func- 

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

 stages of some of the higher forms. They are found 

 also characterising the lowest forms of vegetable life, 

 giving to them the means of spontaneous locomotion, 

 which renders them liable to be mistaken for animals. 

 They consist of prolongations 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 ilexure, 

 followed by a rapid resuming of the perpendicular ; 

 which is however immediately succeeded by like bond- 

 ings and straightenings in alternate gradation. The 

 simplest condition of this movement is that in which a 

 single cilium only exists, by whose successive lash-like 

 beats upon the surrounding water, the animal is rowed 

 along as a boat through the sea. But far more com- 

 monly cilia are arranged in rows, or in many series of 

 rows, in which case 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 



