Comparative Ultrastructure and Systematics of the Colpodida. Structural Conservatism Hypothesis and a Description of Colpoda steinii Maupas*

SYNOPSIS. A brief review of the comparative ultrastructure of several major taxa of ciliated protozoa has disclosed a relationship between the level of biologic organization and the supposed conservatism of cellular structures. The structural conservatism hypothesis proposes that conservation of structure through time is inversely related to the level of biologic organization. If true, this hypothesis will provide a rationale for future systematic investigation of ciliates. Comparative analysis of the fine structure of species of the ciliate order Colpodida will be used as a detailed test of the structural conservatism hypothesis. In this report, the ultrastructure of Colpoda steinii Maupas is described and compared to previous descriptions of colpodids. A new terminology for ciliate ultrastructures is proposed which avoids the ambiguities of the present, taxonomically oriented one.