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.