Why has the 'miracidium' of Notocotylidae (Trematoda: Digenea) lost all stage-specific traits?

Digeneans of the family Notocotylidae differ from other digeneans in their peculiar eggs. The eggs feature a pair of long filaments extending from their poles, and their contents differ significantly from what we expect to observe in the eggs of digeneans. Instead of a ciliated miracidium larva, the notocotylid egg contains a tiny few-celled mother sporocyst. This sporocyst infects a snail host following accidental ingestion of the egg. Previous transmission electron microscope studies elucidated the structure of the sporocyst and proposed a mechanism of the injection relying on the structure called the opercular cord, but the data were insufficient to describe certain details. Here, we present ultrastructural data on the egg of Paramonostomum, clarifying the nature of the injection apparatus and providing the missing details of the in-egg mother sporocyst structure. We found the opercular cord to be part of the eggshell associated with the vitelline membrane. We also elucidated the intertwining connection between the germ cells and the sporocyst's tegument and explored 'nuage' granules present in the cytoplasm of the germ cells, providing evidence for identification of these cells as the germ ones. We discuss our findings in the context of transition of miracidia to a passive strategy of snail infection.