Ultrastructural and cytochemical observations of the spherites in a parasite trematoda larva.

Concentric laminated concretions, termed 'spherites', were found in the parenchymal cells and in the lumen of the collecting vessels of the trematode Meigymnophallus minutus, parasite of a lamellibranch mollusc. In the initial stage, the spherites are surrounded by a membrane and located in close proximity to the endoplasmic reticulum and Golgi complexes, increasing in size with age and by deposit of concentric layers. Later, they migrate and are released into the lumen of the collecting vessels. These structures contain high concentrations of calcium and phosphorus. The organic matrix is mainly composed of protein, lipids, acid phosphatase and glycoproteins as demonstrated by several cytochemical staining methods. It is possible that these calcareous concretions play an important role in the excretory system of the adult trematode, the life cycle of which takes place within the mantle of the host mollusc.