THREE DIVISIONS OF INFUSORIA. 21 



to designate "a class of microscopic animals not 

 furnished with either vessels or nerves, but exhibiting 

 internal spherical cavities, motion effected by means 

 of cilia, or variable processes formed of the sub- 

 stance of the body, true legs being absent." The 

 objection to this definition is, that it to some extent 

 represents theories which may not be true. That 

 nerves are absent all throwjh the class is an as- 

 sumption founded merely upon the negative evidence 

 of their not having been discovered, and the com- 

 plete absence of "vessels" is still a contested point. 

 In the last edition of "Pritchard's Infusoria," to 

 which some of our ablest naturalists have contributed, 

 after separating two groups, the Desmids and the 

 Diatoms, as belonging to the vegetable world, the 

 remainder of the original family of infusoria are 

 classified as Phytozoa^ Protozoa, Rotifer a^ and Tar- 

 digrada. We shall explain these hard names imme- 

 diately, first remarking that the Desmids and the 

 Diatoms, concerning whom we do not intend to 

 speak in these pages, are the names of two groups, 

 one distinctly vegetable, while the other, although 

 generally considered so, is yet held by some au- 

 thorities to be in reality animal. The Desmids 

 occur very commonly in water. We have some 

 among our Confervse. They are mostly brilliant 

 green, and often take forms of a more angular 

 and crystalline character than are exhibited by 



