598 



THE WONDERS OF GEOLOGY. Lect. VI. 



processes resemble very slender fibres or hairs, and are 

 only visible under a powerful microscope. They are 

 situated on parts habitually in contact with water or 

 some other fluid, and possess the power of performing 

 a rotatory or circular oscillation with great rapidity, 

 by which they produce currents and eddies in the sur- 

 rounding fluid. 



When a drop of water containing Infusoria or other 

 animalcules is brought under the microscope, it is seen that 

 as the creatures move along, every particle of foreign mat- 

 ter near them is agitated, a phenomenon indicating eddies in 

 the water. When the Infusoria remain stationary, the cur- 

 rents are more distinct, and evidently take certain direc- 

 tions, causing the particles of matter to run in a stream to 

 and from the animal. If a very high magnifying power be 

 employed, transparent filaments will be distinguished pro- 

 jecting from the surface of the Animalcules, and moving 

 with extreme rapidity. These are the Cilia, which serve 

 to assist in progression, and when the animal is stationary, 

 impel the water in currents through the cavities and 

 tubes on which they are distributed: these must not be 

 mistaken for the tentacula, or feelers, for they are fringes of 

 delicate fibres investing those instruments, and the internal 

 surfaces of other organs. The Cilia are so minute, that their 

 outward form, position, and the direction of their motions 

 only can be detected, their internal structure eluding obser- 

 vation. In the simplest animal organisms they are the 

 organs by which motion and respiration are effected, and the 

 food necessary for nutrition obtained. But they exist also 

 in Man and the higher orders of vertebrata, and are the 

 instruments by which many of the most important functions 

 of the animal economy are performed. As we cannot 

 separate the idea of muscular fibre from animal motion, it 

 is conjectured that the Cilia are impelled by definitely 

 arranged muscles : and Ehrenberg believes he has detected 



