INFUSORIA. 



19 



The external organs of these animals are few and simple. 

 In all, except the lowest forms, the mouth is surrounded 

 by rows of strong cilia, by the vigorous vibrations of 

 which currents are perpetually formed in the water, 

 which bring to the entrance of the stomach whatever 

 particles of matter suitable for food there may be float- 

 ing about. Besides these, the whole surface of the body 

 is, in many species, clothed with delicate cilia, which 

 act as paddles to row the animal rapidly along. A few of 

 the more highly organised genera are furnished with 

 bristles, styles, or hooks, which appear to be merely cilia 

 more than usually developed and deprived of vibratile 

 power. They serve as instruments of locomotion, for 

 crawling, or climbing about aquatic plants. In many 

 species we see a red speck, which is probably an organ of 

 sight in a very rudimental condition j perhaps possessing 

 a sensibility to the presence of light without distinct vision. 



Some of the members of the class are protected by a 

 shell formed of silex, or the substance of flint. These 

 shells may be considered as indestructible, and they are 

 found in a fossil state, the memorials of Infusoria which 

 existed in former ages in multitudes that defy calculation, 

 and almost exceed belief. There is in Bohemia a moun- 

 tain composed of a substance which, from its use in the 

 arts, has long been known by the name of polishing slate. 

 Professor Ehrenberg has found this substance to be en- 

 tirely composed of the shells of fossil Infusoria, the genera 

 and species of which can even yet be distinguished. Of 

 these, he computes that 41,000,000,000 are contained in 

 every cubic inch of a stratum fourteen feet thick. On 

 the shores of certain lakes in Sweden a fine powder is 



