110 THE MECHANICAL FUNCTIONS. 



notion we can form of the solid organic attach- 

 ment of such appendage ; and we can have no 

 conception of organization extending through the 

 medium of a fluid, or of any substance which, like 

 a fluid, admits of the continual displacement of its 

 parts. But the difficulty is soon explained when 

 we examine with more attention the construction 

 of the apparent teeth of this wheel ; and find that 

 each tooth is, in fact, composed of a group of cilia 

 ^,* in different states of extension (as 



OK) ^ 



shown in fig. 36*) ; and that their 

 real motion is not in the direction 

 of the plane of the disk, but at 

 right angles to it. Each group contains cilia in 

 different states of elongation and contraction, com- 

 posing together a pyramidal figure, the apex of 

 which appears to be continually advancing, not 

 from the real progressive motion of the longest of 

 the cilia, but from each, in turn, becoming by its 

 elongation the highest of the group, as it stretches 

 out to its full extent, and afterwards folds itself up 

 so as to take its place among the shortest. As 

 these changes take place in each cilium, in regular 

 succession, the pyramid appears to advance, like 

 a succession of waves, which being instinctively 

 followed by the eye, produce the illusion of a pro- 

 gressive translation of the same object. t 



The action of the cilia is frequently continued 

 for some time after the apparent death of the 

 animal ; and is even persisted in in very small 

 parts that have been separated from the body ; a 

 fact which shows that these motions may be per- 



t See Dr. Farre's paper on Ciliobranchiate Polypi, Phil. Trans. 

 1834,410. 



