Natural History of the Animalcules. 105 
satisfactorily to itself, the various functions performed 
by the skeletal, nervous, muscular, circulatory, respira- 
tory, digestive and reproductive systems in the higher 
forms. The one cell is fun6lionally the exaCt represen- 
tative of all these various systems, and, in each indivi- 
dual life among the higher organisms, such a simple 
cell — but one already potentially differentiated — has given 
origin to such systems. 
It will already have been seen that, from the difficulty 
that is met with in distinguishing many minute forms which 
may be plants or animals, it is impossible to limit defi- 
nitely the extent of the group of the Animalcules, since 
many of these doubtful forms will probably be ultimately 
determined to be animals — in faCt, many of them are so 
determined at present by observers or writers of more or 
less repute, although, generally, a good deal of uncer- 
tainty exists in the matter. The indefiniteness that, on 
this account, attends the application of the term 
Animalcule, is one, however, that will gradually be 
lessened, and possibly removed, as the methods of 
research and the instruments for investigation are more 
perfe6ted ; when, also, the land of the doubtful organisms 
will be doomed for division between the hostile hosts of 
the undoubted plants and animals. 
The term Animalcule is often restricted to one group 
of simple organisms in which the surface is more or less 
covered by those vibratile hair-like or whip-like exten- 
sions of the protoplasm, known as cilia or flagella ) by 
means of which the organism is able to propel itself 
rapidly through the water in which it lives — a group 
otherwise known as the Infusoria, from the presence 
of many of its forms in infusions of animal and, vege- 
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