Fine Structure of Triactinomyxon Early Stages and Sporogony: Myxosporean and Actinosporean Features Compared

. The first ultrastructural study of the actinosporean genus Triactinomyxon was carried out on Triactinomyxon legeri from the intestinal epithelium of Tubifex tubifex. The developmental cycle starts with bi‐ and uninucleate cells. We propose that these cells may be an early proliferative phase of the cycle and may unite to give rise to the four‐cell stage, initiating pansporoblast formation. Valvogenic cells transform in the long stylus and anchor‐like projections of the spore. In the capsulogenic cells, the primordium of the polar capsules originates as a simple, dense, club‐shaped structure not observed in other actinosporeans. In all other respects, actinosporean ultrastructure follows more or less similar patterns. Comparison of actinosporean and myxosporean species gives evidence of considerable structural similarity, exemplified in both classes by the occurrence of cell junctions in their multicellular spores, identical polar capsules and their morphogenesis, cell‐in‐cell condition, pansporoblast formation, and presence of dense bodies (sporoplasmosomes) primarily in the sporoplasm. This unity of patterns speaks in favor of the postulated actinosporean‐myxosporean transformation, which warrants further study.