INFUSORIA. 411 



information to our former meagre knowledge of these in- 

 teresting forms of organic life. 



Some Infusoria undergo a process of encystation before 

 reproduction by fissure ; that is, they become coated with 

 a secretion of gelatinous matter, which gradually hardens 

 so as to enclose the body in a "cyst." According to 

 Stein, the process of encystation is sometimes followed by 

 a remarkable succession of phenomena, such as have been 

 observed to occur in the case of Vorticella microstomcu 

 An old Vorticella loses or retracts its cilia, becomes en- 

 cysted, and finally drops off its stalk. The cyst may either 

 burst and discharge its contents, or become wholly 

 changed into an Acineta-form body. The latter may sub- 

 sequently develop a foot-stalk, assume the appearance of 

 a Podophyra, or even that of the Acineta tuberosa, 

 Plate III. No. 68. In either case, the band-like nucleus 

 becomes transformed into a peculiar ovate body, somewhat 

 like Nos. 71 or 73, the narrow end of which is provided 

 with a circlet of vibratile cilia, and a mouth leading into 

 an internal cavity, with a contractile vesicle in its interior, 

 Relations, somewhat simi- 

 lar to those which connect 

 Vorticella and A cineta, have 

 been stated to exist between 

 other families, as Aspidisca 

 or Trichoda,and Oxytricha, 

 Plate III. No. 71. 



BACTERIA. The remark- 

 able complex bodies bac- 

 teria or bacilli, are amongst 



the most minute forms of Fia 218 ._ Ba cteria of various foam. 



organic life with which the 



microscope has to deal. It has been conclusively shown 

 that bacteria are productive of various kinds of ferments 

 and diseases in the animal body. They require for their 

 development a free supply of oxygen, and they thrive best 

 in albuminoid fluids. Whether decomposition of albu- 

 minoid matters is directly occasioned by their life pro- 

 cesses, or whether they generate a ferment which induces 

 fermentation, is not positively known. It is quite clear 

 however, from the investigations of Pasteur, Cohn, 



