The Protozoa

Abstract

This group contains the animals with the most simple body structure. All animals which, when fully developed, have bodies consisting of one cell only (i.e. one nucleus and the cytoplasm under its control) are Protozoa. It is difficult, however, to give a simple definition which describes all the forms which are contained in the group for two reasons. First, because there are some members with complicated bodies and several nuclei; these, however, are not truly multicellular like higher animals, and they are obviously closely related to other simpler Protozoa. (Some zoologists describe all Protozoa as ‘ non-cellular ’ rather than ‘ uni-cellular ’, but it is impossible to discuss the implications of this without having a detailed knowledge of the group.) Secondly, many lowly one-celled plants (Protophyta) are very similar to Protozoa, and only differ in the way they obtain their food. The Protozoa, being animals, feed by taking in complex food substances (i.e. by eating other animals, plants, or their decaying remains) which were ultimately produced by growing plants. The Protophyta feed for the most part like typical green plants — they contain chlorophyll which enables them to synthesize carbohydrates from carbon dioxide and water. There are ‘animals’ which are intermediate between the Protozoa and the Protophyta like some Euglenae which sometimes live like animals and at other times contain chlorophyll and live like plants.