Infusoria: What are they ? 45
Radiolaria, &c), in no 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 will
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" (Paramaecium), 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