Answer: Blastocystis hominis
Anonymous said it well: "Blastocystis. There are different morphological forms of the organism and subtypes. Host specificity is poor. Many experts have dropped the "hominis".
I will look forward to further studies to determine if we keep the species designation. Meanwhile, I will continue to use the nomenclature used by the U.S. Centers for Disease Control and Prevention on their excellent Parasite web site, DPDx
Blastocystis can be identified in stool specimens by its characteristic cyst morphology, with central vacuole and peripheral nuclei (below).
Trichrome Stain, 1000x original magnification
This "thick-walled cyst" is thought to be the form of the organism that is transmitted to others. However, much is unknown about Blastocystis, including its life cycle and ability to cause disease. Recent phylogenetic studies have placed this organism in the informal group, the stramenopiles, which include water molds, diatoms, and brown algae. I'm sure there will be much more information to come on this fascinating protozoan parasite.