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Miracidium is the larva of digeneans that serves to infect mollusks (first intermediate hosts). Many digeneans possess miracidia that use active strategy of infection. These larvae swim by ciliary action to the specific mollusks and penetrate into them. Miracidia of other digeneans follow an essentially different, passive way of infection: they rest in the eggshells and the mollusks become infected only after they ingest the eggs with larvae inside. The differences in strategies of infection are reflected in contrasting morphologies of the two types of miracidia. "Passive" ones are always smaller and "simplified". However, very little is known about details behind this reduction. This is due to the evident lack of ulstrastructural data on "passive" forms. Here we present the TEMreconstruction of P. squamatus "passive" miracidium and compare it with the well-known structure of "active" forms.

We elucidate the morphology of the miracidia with passive strategy of infection. In contrast to the well‐studied “active” free‐swimming larvae (e.g., those of Schistosoma, Fasciola, Echinostoma), “passive” miracidia do not search for their hosts in the external environment. The infection occurs only after the mollusk ingests the eggs with the larvae. The miracidia of this type are extremely miniaturized organisms whose somatic elements are reduced compared to the “active” forms. The details of the structure are unknown for the majority of taxa with “passive” larvae. Here, we provide the first description of a gymnophalloid miracidium based on ultrastructural data. The larva of Parvatrema affinis Jameson & Nicoll, 1913 consists of 21 cells. Its nervous and excretory systems are reduced to the extreme level. Its penetration apparatus includes two crystalloid gland‐cells, unique among digeneans. The “true” epithelium of its body wall is a novelty never described for any other miracidium. We compare the structure of gymnophalloid and bucephaloid miracidia trying to determine possible ancestral features of the larvae in this digenean lineage and trends of their evolution.

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The article provides characteristics of a new parasite, Crithidia dobrovolskii sp.n., which was isolated from the tachinid fly captured in the Leningrad Region of Russia. The presented description of Crithidia dobrovolskii sp.n. is based upon light microscopic, ultrastructural, and molecular phylogenetic data. Molecular phylogenetic analyses of SSU rRNA gene and GAPDH gene sequences have demonstrated that the new species is most closely related to Crithidia fasciculata.