RESUMO
We performed a detailed ultrastructural reconstruction of the "passive" miracidium of Derogenes varicus Muller, 1784 , a species from the Hemiurata group. The miracidium is highly miniaturized and simplified in comparison with the "active" miracidia. For the first time we elucidated the nature of the spines on the surface of the hemiuroid larva: they are derivatives of the epithelial plates. The anterior end of the larva is equipped with three epithelial plates that bear both spines and cilia. The major part of the miracidial surface is formed by the tegument. The nervous and excretory systems of the D. varicus miracidium are extremely reduced. Single undifferentiated cell comprises the germinal material of the miracidium. We discuss the trends of evolution of hemiuroid miracidia that are associated with the transition to passive strategy of infection.