r. 



4 INFUSORIA. 



minute (in most cases, organised) beings, 

 chiefly consisting of 



A. Rliizopoda; 



B. Unicellular and other Algse ; 



C. Embryonic forms ; and, lastly, 



D. True Infusoria. 



The true nature of many of the Infusoria 

 proper is still a disputed question. 



According to Dujardin, their bodies are com- 

 posed of a gelatinous substance, similar to that 

 which we find among the Rhizopoda. By Siebold, 

 Meyen, Kolliker, and others, they have been re- 

 garded as unicellular animals ; a view of their 

 nature which certainly does not appear to be con- 

 firmed by the examination of the more highly or- 

 ganised forms. Agassiz, on the other hand, has 

 endeavoured to get rid of the entire gi'oup of In- 

 fusoria by assigning higher positions in the animal 

 kinofdom to those of its members whose non-em- 

 bryonic nature would seem to be fully established. 



In the midst of so many conflicting opinions, 

 the following course has seemed to us most worthy 

 of adoption. 



Combining the results of our ovm recent obser- 

 vations with those of the more elaborate inves- 

 tigations of Claparede, Lachmann, and others, we 

 shall, with some limitations, adopt the views of the 

 last-mentioned authors, and define the InfusoriasiS 

 Animals belonging to the department of Proto- 

 zoa, provided with a mouth and rudimentai^ 

 digestive apparatus; their bodies usually con- 

 sisting of three distinct layers, the outer of vjhich 

 is, in most cases, furnished vjith a variable num- 

 ber of cilia. 



