236 INFUSORIA. 



plication takes place exclusively by division, until exhaustion renders 

 another conjugation imperative. 



Fission in the majority of Infusoria is transverse, so that the 

 anterior part has to form a new anus, and the posterior part a new 

 mouth and the associated circlet of cilia. In other cases the division 

 is longitudinal, and it may even take place diagonally. Nor is it by 

 any means necessary that the two parts should be equal ; indeed, the 

 one is often so very minute that it looks like a little bud. When the 

 division is repeated in quick succession, the resulting organisms are 

 of extraordinary minuteness, which is particularly seen in the fixed 

 forms as long as the progeny remain connected together in a group. 



In many species the division only takes place after the mother- 

 animal has drawn itself together into a ball and donned a capsule. 

 Sometimes two, sometimes four, six, or eight young Infusoria result, 

 which remain within the capsule till they break through it, and con- 

 tinue their life outside. 



This encystation occurs for other purposes than that of reproduc- 

 tion. At other times also the Infusoria have the power of secreting a 

 cyst and passing into a quiescent state. This may be observed on 

 approaching scarcity of water, or when the environment is in some 

 way abnormal. It takes place indifferently in adult and young forms. 

 Protected by this cyst, which is often thick and very resisting, these 

 very delicate creatures can survive complete dessication. They may 

 be kept, like seeds or eggs of worms, for years, and yet, on the appli- 

 cation of water, often recover their full vital energy within a few 

 hours, and break through the capsule. The importance of this for the 

 Infusoria is very obvious. It is not only a means of preservation, but 

 of propagation ; for the wind, passing over the dried up ditches, lifts 

 the capsules and carries them to great distances, dropping them again 

 in most varied situations on leaves, moss, and bark, in chinks, 

 bowls, and infusions. Such germs have been repeatedly demonstrated 

 by Ehrenberg and others in atmospheric dust, by passing the air 

 through pure water. We do not require to point out how this habit 

 is specially favourable to the parasitic occurrence of these animals. 



If the Infusoria be carried to, or in any way reach, a situation and 

 environment which fulfil the conditions of their life, they multiply so 

 rapidly that their number increases in a short time to an almost in- 

 credible degree. Theoretical calculations which have been made give 

 results on this matter not less astounding than those above quoted for 

 the Schizomycetes. 



In reference to the manifold and fundamental differences between 

 the various forms of Infusoria, we may note that the group is generally 

 divided into "Flagellate" with a flagellum, and "Ciliata" with cilia. 



