Most organisms that use intracellular digestion belong to Kingdom Protista, such as amoeba and paramecium. Examples include "Paramecium", "Stentors", and "Vorticella". Their bodies are smaller than a single-celled "Paramecium". Paramecium aurelia are unicellular organisms belonging to the genus "Paramecium" of the phylum Ciliophora. Familiar examples of protists with a pellicle are the euglenoids and the ciliate "Paramecium". Studies of "Paramecium" have contributed to our understanding of the function of meiosis. GTP and ATP are chemorepellents in micro-molar concentrations to both "Tetrahymena" and "Paramecium". Species of "Paramecium" range in size from in length. Without the rejuvenating effects of autogamy or conjugation a "Paramecium" ages and dies. In "Paramecium aurelia", a cryptic species complex was discovered by observation. These nuclear divisions are observed mainly when the Paramecium aurelia is put under nutritional stress. "Paramecium aurelia" is the most commonly studied protozoan for autogamy. Research shows that "Paramecium aurelia" undergo autogamy synchronously with other individuals of the same species. "Paramecium" reproduces asexually, by binary fission. The "Paramecium" spirals through the water as it progresses.