42 NOTES ON 
SECTION IT. 
ON ANIMALCULES OR INFUSORIA. 
THE two succeeding chapters are devoted to 
that minute class of living beings denominated 
animalcules. This term admits of great latitude. 
It is not confined to that numerous tribe of 
aquatic animals which are wholly invisible to 
unassisted vision, but is applied to all whose 
members require the aid of a microscope to 
render them manifest. Some have preferred 
the term infusoria, they being always found in 
infusions of vegetable or other organized mat- 
ter, and have defined them as mere active gela- 
tinous matter, devoid of any muscular, diges- 
tive, or nervous system. A careful examination, 
however, under a good instrument, will show 
that all of them are possessed of digestive 
organs, and many, especially some species of 
the Vorticella, are highly organized. At the 
same time, it cannot be denied that some func- 
tions, which in the larger animals require 
distinct organs for their performance, may in 
these be effected by a peculiar conformation of 
the integuments which envelope them, their 
