INFUSORIA. gi 
infusions, vegetable or animal. With the assistance of a microscope 
the reader may, with very little trouble, afford himself the pleasure of 
studying these animals. It is only necessary to place some organic 
débris—the white of an egg, or some chopped hay, for example—in 
a bottle with a large mouth, filled with water, and expose it to the 
light and air, and it will be found in the course of a few days to 
swarm with infusorial life. Certain reagents, as phosphate of soda, 
the phosphates, nitrates, or oxalates of ammonia, or carbonate of 
soda, added to these infusions, favour the development of Infusoria. 
So much for the medium in which they live, move, and have their 
being. Let us pass on to their organisation. We have already dwelt 
on their extreme minuteness ; their mean size may be the fifth of a 
line, or the sixtieth part of an inch; the largest species scarcely reveal 
themselves to the naked eye. They are generally colourless ; some 
of them are, nevertheless, green, blue, red, brown, and even blackish. 
Seen under the microscope, they appear either transparent and naked, 
or invested with an envelope more or less resistent, which is homo- 
genous, diaphanous, elastic, contractile, and apparently destitute of 
every kind of organisation. They are of every imaginable shape. 
Some of those most frequently met with, and which from their size 
attract the most attention from observers, are furnished with vz6ratile 
cilia, which either cover the whole body, or are attached to certain 
portions of it, acting as paddles. These organs are evidently intended 
to propel the animal from one place to another, while at other times 
certain of them appear to be employed in conveying food to the 
mouth. Some Infusoria are without these cilia, having only one or 
many very slender filaments or flage//a, the undulating movement ot 
which suffices to determine their progression through the liquid 
which surrounds them. 
Authors who have written on the Infusoria have sometimes, like 
Leuwenhoek and Ehrenberg, attributed to them a very complex 
structure. Others, like Miiller, Cuvier, Claparade, Lamarck, Stein, 
and all recent writers, have considered them to be gifted with an 
organisation extremely simple. 
Some of the species are indeed of very lowly organisation ; while 
again many of them are among the most highly organised of the 
Protoza, for here we find the first appearance of a well differentiated 
alimentary system. Indeed, the digestive system of the Infusoria has 
been the subject of numerous observations, and was at one time the 
subject of very animated discussions. In the inferior members of the 
class, which comprehends the very smallest animalcules, it has not 
