256 EVENINGS AT THE MICROSCOPE 



utterly unable to conceive how such a movement could 

 consist with parts maintaining an organic connection be- 

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

 depending on the nature of ciliary movement, 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 functions, 

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

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

 acterizing the lowest forms of vegetable life, giving to 

 them the means of spontaneous locomotion, which ren- 

 ders them liable to be mistaken for animals. They con- 

 sist 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 flexure, followed 

 by a rapid resuming of the perpendicular; which is, how- 

 ever, immediately succeeded by like bendings and straight- 

 enings 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 sur- 

 rounding water the animal is rowed along as a boat 

 through the sea. But far more commonly cilia are ar- 

 ranged 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 be- 

 gins 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 completed its 



