INFUSORIA. 91 



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 

 de'bris — 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 fii'th 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 vibratilc 

 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 flagella, the undulating movement ot 

 which sufiices 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 



