FINE-STRUCTURE OF THE MATURE PLASMODIUM OF INTOSHIA-VARIABILI (PHYLUMORTHONECTIDA), A PARASITE OF THE PLATYHELMINTH MACRORHYNCHUS-CROCEA

The fine structure of the mature plasmodia of Intoshia variabili, foun d in the kalyptorhynch flatworm Macrorhynchus crocea, was studied with transmission electron microscopy. As the plasmodium matures it forms long finger-like extensions inside the host. These extensions eventual ly take the place of the genital system of the host turbellarian. The mature plasmodium is separated from the host tissues by a continuous m embrane. The entire plasmodium is surrounded by modified host cells, w hich are thin and overlapping. These cells also have a rather homogeno us cytoplasm with limited endoplasmic reticulum. The surface of the pl asmodium is mostly smooth and has numerous small extensions. Within th e cytoplasm of the plasmodium, different stages of development of both males and females could be observed. The cytoplasm also contains nume rous small nuclei with distinct nucleoli. Scattered throughout the cyt oplasm, in addition to nuclei, are mitochondria, various granules, mul tivesiculate bodies with lamellar structures, and a large number of sm all vesicles. The nature of the formation of the plasmodium is discuss ed. (C) 1998 The Royal Swedish Academy of Sciences. Published by Elsev ier Science Ltd. All rights reserved.