A Redescription of Conchophthirus curtus Engelmann, 1862 (Protozoa, Ciliatea)1

SYNOPSIS. The structure of Conchophthirus curtus is described in detail, based on observations by light microscopy, transmission electron microscopy and scanning electron microscopy. Characters of taxonomic importance are described from study of specimens representative of populations of the ciliate collected from 5 localities in Illinois. Variations within and between populations are indicated. New observations include a thoro description of the buccal cavity with its 3 polykineties, a haplokinety, and a deep kinetosomal unit. The somatic ciliature is differentiated into locomotor, thigmotactic, vestibular and caudal regions. In discussion of the systematic position of Conchophthirus, critical review of its past assignments is included; and comments on both the higher and lower levels of taxonomy of the “thigmotrichs” and their relatives are offered.