Light and electron microscopic studies on Goussia zarnowskii (Jastrzębski, 1982): an intestinal coccidium parasitizing the three‐spined stickleback, Gasterosteus aculeatus (L.)

. The foregut and midgut of sticklebacks, Gasterosteus aculeatus (L.), naturally infected with Goussia zarnowskii (Jastrzębski, 1982) was studied by light and electron microscopy. Trophozoites, meronts, gamonts and young oocysts developed in the intestinal epithelial cells within a parasitophorous vacuole located in an ectoplasmal position and covered by host cell plasmalemma towards the intestinal lumen. A persistent conoid, together with two polar and preconoidal rings, was present in trophozoites, young meronts and also in young macrogamonts. All developmental forms of the parasite, excluding microgamonts, were surrounded by a trimembranous pellicle consisting of an external continuous unit membrane and double‐membranous system of flattened vesicles. The micro‐gamont pellicle was reduced to a single surrounding membrane during the development of the microgamont. The manner of merogony remains unknown. The oocyst wall consisted of a double membrane of host cell origin and at least three additional membranes derived from the endoplasmic reticulum of the zygote. Material from inclusions of double electron‐density, that had occurred formerly at the periphery of the macrogamont cytoplasm, was disposed outside of the pellicle between the newly‐created oocyst wall membranes. The function of this deposit and way it passes across the pellicle membranes remains unknown. The oocyst is surrounded by the membranes of the parasitophorous vacuole and the host cell plasmalemma detached from the surface of the intestinal epithelium facing the intestinal lumen.