ering of cilia Order _Heterotrichida_
+ adoral zone

c. With cilia on ventral side Order _Hypotrichida_
+ adoral zone

d. With cilia in region of adoral Order _Peritrichida_
zone, and about mouth only

KEY TO FAMILIES OF THE HOLOTRICHIDA.

A. Mouth closed except during food 1
ingestion; no undulating membrane

Mouth always open; with undulating 2
membrane

1. _Gymnostomina_.

a. Mouth terminal or subterminal. Family _Enchelinidae_
Food is swallowed and not
introduced by currents

b. Mouth terminal or subterminal; Family _Trachelinidae_
body frequently drawn out into
long process; mouth may have
specialized framework.

c. Mouth central or posterior; Family _Chlamydodontidae_
pharynx with supporting
framework

2. _Trichostomina_.

a. Mouth anterior or central; Family _Chiliferidae_
pharynx short or absent;
peristomial depression faint
or absent

b. Mouth central; pharynx long, Family _Urocentridae_
tubular; cilia in two broad
zones

c. Mouth posterior; form Family _Microthoracidae_
asymmetrical; cilia dispersed
or limited to oral region

d. Mouth anterior or central. _Paramoecidae_
Peristomial depression (One genus, _Paramoecium_)
clearly marked.

e. Mouth at end of long peristome Family _Pleuronemidae_
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 _Isotrichidae_
posterior; cilia uniform.
Parasites in ruminants.

g. Mouth absent; body vermiform, Family _Opalinidae_
cilia uniform. Usually
parasites.

KEY TO MARINE GENERA OF ENCHELINIDAE

Diagnostic characters: Form ellipsoid or ovoid; the mouth is
invariably terminal and is usually round--more rarely slit-formed; it
is closed except when food is taken. An oesophagus when present is a
short, invariably non-ciliated tube which is usually surrounded by
a more or less clearly defined buccal armature. The anus is usually
terminal. Large food particles are swallowed, never introduced by
currents.

1.