The ultrastructural cytology of Bacillidium strictum (Léger and Hesse, 1916) Jírovec, 1936 (Microspora,...

The ultrastructural cytology of a microsporidium identified as Bacillidium strictum (Léger and Hesse, 1916) is described. The host was one of 28 investigated specimens of the oligochaete Limnodrilus hoffmeisteri, collected from a small stream in southern Sweden, not the type host Lumbriculus variegatus. The microsporidium occurred together with an eugregarine of the genus Rhynchocystis and an actinomyxidian of the genus Echinactinomyxidium. The 2.0-2.2 × 11.6-13.3 μ great spores (fixed and stained) were slightly smaller than the size indicated in the description. They occurred in spherical, hypertrophic haemocytes. The exospore, which is approximately as thick as the endospore, has a wide, electron-dense basal layer, an external layer resembling a double-membrane, and fibrous, periodically arranged surface projections. The double-layer remains attached to the basal layer. The polaroplast has an anterior region with closely packed lamellae and a posterior section with wider lamellae or tubules. The polar filament has a wide, straight anterior part, a zone with successively reduced diameter, and a posterior, narrow section, which turns in anterior direction, forms one or two coils around the diplokaryon and the wide filament, and ends in the proximity of the polaroplast. Approximately 100 nm wide tubules, exhibiting exospore construction, are initiated at the onset of sporogony, and persist also in the proximity of mature spores. The species status of the microsporidium and the ultrastructural characteristics of the Bacillidium-like microsporidia are discussed.