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The development of Myxobolus macrocapsularis Reuss, 1906, a myxosporean parasite of the gills of common bream Abramis brama L., was studied in experimentally infected oligochaetes. In 3 experiments uninfected Tubifex tubifex Muller and Limnodrilus hoffmeisteri (Claparéde) were exposed to mature myxospores of M. macrocapsularis. In all experiments, typical triactinospores developed in T. tubifex specimens but no infection was found in L. hoffmeisteri. Triactinospores were released from oligochaetes 66 to 99 d after initial exposure. At that time pansporocysts containing 8 triactinospores were located in the gut epithelium of experimental oligochaetes, but free actinosporean stages were also found in the gut lumen of the oligochaetes. Each triactinospore had 3 pyriform polar capsules and a barrel-shaped sporoplasm with 32 secondary cells. The spore body joined the 3 caudal projections with a stout style.

Sporocarp morphology and zygospore ontogeny of Endogone pisiformis were examined with light and electron microscopy. Sporocarps consist of a peridium of thick-walled, dextrinoid, tapering hyphae which enclose a gleba of zygosporangia and thick- and thin-walled hyphae. Zygosporangia develop from the fusion of paired, apposed gametangia. The zygosporangial wall consists of a thin outer layer which is disrupted by the much thicker, expanding inner zygosporangial wall layer, within which the zygospore develops. Young zygospores are irregular in shape and have thin walls. As the zygospore wall increases in thickness, it remains closely associated with the wall of the parent zygosporangium and becomes more globose to ovoid in shape. Zygosporangial and zygospore walls at all stages of development contrast in staining properties with both light and electron microscopy. Gametangial septa are multiperforate, and glebal hyphae have infrequent septa with single, central perforations. Young zygospores contain apparent glycogen, which is generally absent in mature spores, and the sporoplasm contains two types of lipid globules which differ in their staining properties.