Infusoria : What are they ? 45 



Radiolaria, &c.), in uo way connected with each other, and 

 whose amalgamation with the Infusoria can no longer be 

 seriously entertained. 



The Infusoria, as now delimited, may be defined as micro- 

 scopic, unicellular animals, or Protozoa, whose locomotive 

 or prehensile organs take the form of one or more long 

 whip -like appendages, or " flagella " ; of variously developed 

 fine hair-like appendages, or " cilia" ; or of a more or less 

 complete system of extensile and retractile tubular structures, 

 or "tentacula." It is with reference to these three modi- 

 fications of their appendages that the Infusoria are con- 

 veniently divided into the three primary sections, or classes, 

 of the Flagellata, the Ciliata, and the Tentaculifera, to one 

 of which groups the student must relegate the subject or 

 subjects of his investigations as a preliminary step towards 

 their specific identification. Out of these several classes the 

 microscopist entering upon an examination of this fascinating 

 organic series will do well, perhaps, to concentrate his atten- 

 tion firstly upon the class Ciliata, the majority of its members 

 being of appreciable size and adapted for study and identifi- 

 cation with comparatively low powers of the compound 

 microscope. Confining his energies to this class, he wiU 

 find that the organisms he collects conform to one out of 

 four structural formulae, which serve as bases for the sub- 

 division of the class into as many subordinate groups or 

 orders. Thus, in one very extensive series, the fine hairs or 

 cilia are closely alike in size and character, and are distri- 

 buted uniformly over the entire surface of the body. This 

 order, known technically by the title of the Holotricha, is 

 typified by the " Slipper Animalcules " {Paramecium), common 

 in infusions; the "Toothed Animalcules" (Nassula and 

 Prorodon); and the *' Swan Animalcules" (Trachelocerca). 

 In the second order, that of the Heterotricha, the cilia — as 

 the term implies — are of diverse kinds ; those of the general 

 surface of the body being very fine, as in the preceding 

 group, to which, however, is superadded a much larger and 

 stronger series which surrounds or conducts to the oral 

 aperture or mouth. This order includes some of the largest 



