a. Mouth anterior or central; Family _Chiliferidæ_
pharynx short or absent;
peristomial depression faint
or absent
b. Mouth central; pharynx long, Family _Urocentridæ_
tubular; cilia in two broad
zones
c. Mouth posterior; form Family _Microthoracidæ_
asymmetrical; cilia dispersed
or limited to oral region
d. Mouth anterior or central. _Paramoecidæ_
Peristomial depression (One genus, _Paramoecium_)
clearly marked.
e. Mouth at end of long peristome Family _Pleuronemidæ_
running along ventral side;
body dorso-ventrally or
laterally compressed; left edge
of peristome with great, sail-
like undulating membrane
f. Mouth and pharynx distinct, Family _Isotrichidæ_
posterior; cilia uniform.
Parasites in ruminants.
g. Mouth absent; body vermiform, Family _Opalinidæ_
cilia uniform. Usually
parasites.