INFUSORIA. 
19 
The external organs of these animals ai - e 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, whicli is probably an organ of 
sight in a very rudimcntal condition ; 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 sites, or the substance of flint. These 
shells may be considered as indestructible, and they are 
found iu a fossil state, the memorials of Infusoria whicli 
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 
ei eiy cubic inch of a stratum fourteen feet thick. On 
the shores of certain lakes in Sweden a fine powder is 
