INFUSOKIA. 99 



numerable masses of small Infusoria having a reddish shell. Finally, 

 let us add that the solid debris of certain fossil Infusoria, of surprising 

 minuteness, have formed the stone so much used by workers in metal, 

 which is known as tripoli. 



The study of these creatures is intensely interesting to the naturalist, 

 the philosopher, the physician, and the general reader. They have 

 had a great part assigned to them in Nature, as is evident in the forma- 

 tion of certain beds of rock of immense extent, in which the geologist 

 traces their action. 



Our earliest knowledge of the Infusoria is traceable to the seven- 

 teenth century; to the celebrated naturalist, Leuwenhoek, we are 

 indebted for their discovery. On the 24th of April, 1676, this 

 observer saw for the first time some infusorial animalcules. Fifty 

 years later, Baker and Trembley studied them anew. In 1752, Hill 

 essayed the first attempt at their classification. In 1764, Wiesberg 

 gave them the name of Infusoria, because he found them in such 

 great abundance in animal and vegetable infusions. Miiller pub- 

 lished a special book upon them. 



From that time the Infusoria have been considered as forming a 

 special group among the radiate animals ; afterwards, in the pages of 

 Baer and of De Blainville, we see in these creatures, so imperfect in 

 appearance, only the indeterminate prototype of other classes. But 

 ideas changed altogether respecting them when microscopes of great 

 power, and armed with achromatic lens, were employed in their study. 

 Thanks to the labours of Ehrenberg and Dujardin, we have arrived at 

 a better comprehension of the organization of these infinitely small 

 beings. Naturalists have established, with more exactness, the limits 

 of the zoological group to which they belong. 



Some stagnant waters are so filled with Infusoria that it is only 

 necessary to dip at random into the liquid medium to procure them in 

 abundance. In other waters they form a bed, occupying the whole 

 basin. In general, it is necessary to search for them where the water 

 is calm, and occupied by vegetation of some kind, such as the confervas, 

 or lemna, &c., in the marshes, and ceramium if in the sea. Certain 

 Infusoria live not only in water, but also in places habitually moist, as 

 among tufts of mosses ; in beds of oscittaria, on moist soil, or on air- 

 damp walls. Others live as parasites on the exterior or in the interior 

 of animals, such as hydra, lombrics, and naiads. Quantities of them 



H 2 



