Fine structure of a tiny gymnophalloid miracidium (Digenea).

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.