Diagnostic characters: The peristome is a narrow furrow which begins,
as a rule, close to the anterior end and runs backward along the
ventral side, to the mouth, which is usually placed between the
middle of the body and the posterior end. A well-developed adoral
zone stretches along the left side of the peristome, and is usually
straight.
1. Body cylindrical; size medium; Genus _Metopus_
peristome long and turns sharply
to the left at the extremity
2. No torsion in the peristome; Genus _Blepharisma_
undulating membrane is confined
to the posterior part of peristome
3. No peristomial torsion; Genus _Spirostomum_
body highly contractile;
no undulating membrane