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 _Enchelinidæ_
Food is swallowed and not
introduced by currents

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

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

2. _Trichostomina_.

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