Observations on the Spores of Pleistophora gigantea (Thélohan, 1895) Swellengrebel, 1911, a Microsporidan Parasite of the Fish Crenilabrus melops*

SYNOPSIS. After 1914 protozoologists have generally agreed that Pleistophora gigantea (Thélohan, 1895) Swellengrebel, 1911, Ichthyosporidium giganteum (Thélohan, 1895) Swarczewsky, 1914, and I. phymogenes Caullery and Mesnil, 1905, are identical. Because no polar filament was found in the spores, however, some authors have followed Swarczewsky in considering this species to be a haplosporidan, while others have persisted in thinking it a microsporidan. Using preserved material that Swellengrebel saved from a tumor on which he based his studies, we have found a polar filament in the spores both with the PAS reaction and with the electron microscpe. This new information removes the only basis for the doubt which some authors have entertained, that Thélohan and Sweliengrebel correctly considered the parasite to belong to the Microsporida. Since Pleistophora gigantea is believed to be identical with I. phymogenes, recently selected by Sprague as type species of genus Ichthyosporidium Caullery and Mesnil, 1905, then Ichthyosporidium, originally assigned to the Haplosporida, must be regarded as a microsporidan genus. Whether it is distinct from all other microsporidan genera is a matter needing further consideration.