INFUSORIA. 47 
Coiey eer elle 
INFUSORIA. 
THE last class of the Protozoa is that of the Zxzfusoria, 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 unnecessary to say 
that if this infusion be examined under the microscope, after 
boiling, nothing will be detected in it,—nothing living, at any 
rate. If examined, however, at the end of a few days’ time—if 
the circumstances have been favourable—a vast number of living 
forms will now be found in it. Amongst these will be found 
several of the members of the present class, and hence the name 
applied to them of Infusorian animalcules, or Zufusoria. 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 eminent observers who hold that the appearance 
of the /zfusoria in this fashion is to be explained upon the 
theory that they have been spontaneously produced out of the 
organic, but dead, matter of the infusion, in opposition to the 
general view that they are derived from pre-existent germs, 
The position of the Jzfusoria is somewhat doubtful, and it 
appears probable that they will ultimately have to be regarded 
as a separate sub-kingdom, or as a branch of a higher sub- 
kingdom (Aznuloida). In the meanwhile it is most convenient 
to retain them in their present place, at the head of the sub- 
kingdom Protozoa. Regarded in this light, the Infusoria pre- 
sent a great advance in structure over all the forms which we 
have hitherto studied—an advance which is especially seen in 
the almost constant presence of a permanent mouth. For this 
