INPUSOEIA AND OTHER ANIMALCULE. 243 



type of Infusoria in general ; but there are other 

 more highly- organized forms in this vast family, 

 which recall sometimes the bell-shaped polypes, or 

 other animals of still more complicated structure. 

 The Rotifera, or wheel-animalcules, which were until 

 lately classed with Infusoria, have been gradually ele- 

 vatedtothe class of TFbrms,andare nowplaced bysome 

 zoologists near the tribe of mites (Acarus). They 

 belong, therefore, to the highest of inferior animals, 

 namely, to the class of Spiders. The Vibrio tritici, 

 an eel-like animalcule, which causes the " ear- 

 cockle/' or the blight, in wheat, has been taken 

 from the class of Infusoria, and placed in that of 

 Helminthes or Entozoa (worms). 



Some infusorial animalcules secrete themselves 

 a covering of hard flint (silica), resembling in 

 this respect the plants which belong to the family 

 of Equisetacce and the Grasses, the epidermis of 

 whose stems contains sometimes as much as 90 per 

 cent, of silica. 



The covering or outer tunic of Infusoria is, 

 then, of two kinds : the one soft and apparently 

 membranous, yielding to the slightest pressure ; the 

 other rigid and hard, having the appearance of a 

 shell, though, from its flexibility and transparent 

 nature, it is more like horn. The microscopic 

 beings belonging to the class of Rhizopoda a class 

 higher than Infusoria present also the latter pe- 



