CILIARY MOVEMENT. 57 



corn when depressed by the wind; and if a number be 

 affected in succession with this motion, the appearance of 

 progressive waves following one another is produced, as when 

 a corn-field is agitated by repeated gusts. When the ciliary 

 motion is taking place in full activity, however, nothing can 

 be distinguished save the whirl of particles in the surround- 

 ing liquid; and it is only when the rate of movement 

 slackens, that the shape and size of the individual filaments, 

 and the manner in which their stroke is made, can be made 

 out. The motion of the cilia is not only quite independent 

 (in all the higher animals at least) of the will of the animal, 

 but is also independent even of the life of the rest of the 

 body ; being seen to continue after the death of the animal, 

 and even going on with perfect regularity in parts separated 

 from the body. Thus, isolated epithelium-cells have been 

 seen to swim about actively in water, by the agency of their 

 cilia, for some hours after their detachment from the mucous 

 membrane of the nose; and the regular movement of cilia 

 has been noticed fifteen days after death, in the body of a 

 tortoise in which putrefaction was already far advanced. In 

 the gills of the Eiver Mussel, which are amongst the best 

 objects for the study of this most curious phenomenon, the 

 movement endures with similar pertinacity. The purpose of 

 this remarkable agency is obviously to propel fluids over the 

 surfaces which are furnished with cilia. We find it taking 

 the most important share in the functions of life among the 

 lowest classes of animals. Thus, in Animalcules of various 

 kinds, the cilia are the sole instruments, not merely for the 

 production of those currents in the water which may bring 

 them the requisite supplies of air and food, but also for pro- 

 pelling their own bodies through the liquid. In most 

 Zoophytes, and in the inferior Mollusks, which pass their 

 lives with little or no change from one spot to another, the 

 motion of the cilia lining the alimentary canal and clothing 

 the gills (where such have a special existence), draws into the 

 mouth the minute currents which serve as food, and also 

 renews the layer of water in contact with the respiratory 

 surface. The gills of Fishes are not furnished with cilia, 

 another provision being .made by muscular action for conti- 

 nually driving fresh streams of water over them ; but the 

 motion may be very well seen upon the gills of the young 



