CHAPTER IIL 



Intusoeia. 



The last class of the Protozoa is that of the Infusoria, so 

 called because of their being frequently developed in organic 

 infusions under the following singular circumstances : If some 

 water be taken, and any animal or vegetable substance be 

 soaked or boiled in it, a solution is formed containing organic 

 matter, or, in other words, an " organic infusion." It is un- 

 necessary to say that if this infusion be examined under the 

 microscope, after boiling, nothing will be detected in it — 

 nothing Uving, at any rate. If examined, however, at the end 

 of a few days' time — if the circumstances have been favor- 

 able — a vast number of living forms will now be found in it. 

 Among these will be found several of the members of the 

 present class, and hence the name applied to them of Infusorian 

 animalcules, or Infusoria. It is unnecessary to enter here 

 into the question how these living beings are produced, since 

 the subject is one of great obscurity, and opinions are still 

 divided upon it. It is sufficient to remark that there are emi- 

 nent observers who hold that the appearance of the Infusoria 

 in this fashion is to be explained upon the theory that they 

 have been spontaneously produced out of the inorganic ma- 

 terials of the infusion, in opposition to the general view that 

 they are derived from preexisterit germs. 



The position of the Infusoria is somewhat doubtful, and 

 it appears probable that they will ultimately have to be re- 

 garded as a separate sub-kingdom, or as a branch of a higher 

 sub-kingdom {Annuloida). In the mean while it is most con- 

 venient to retain them in their present place, at the head of 

 the sub-kingdom Protozoa. Regarded in this light, the In- 

 fusoria present a great advance in structure over aU the 

 forms which we have hitherto studied — an advance which is 

 especially seen in the constant presence of a permanent 



