[Analysis of possible mechanisms of orthonectid emergence from their hosts]

In the present study authors claim that the adult orthonectids can not move through host tissues by themselves. In various species of these enigmatic parasites there are at least two different mechanisms of emission of males and females from the host body. Intoshia linei, the orthonectid from Lineus ruber (Heteronemertini), and Intoshia variabili, the parasite of a flatworm Macrorhynchus crocea, realize the first way of emission. The plasmodium of these species forms tube-like outgrowths, which pierce the host tissues reaching the host body surface. The cytoplasm structure of these outgrowths differs from the cytoplasm of the central mass of plasmodium. Small mitochondria with electron dense matrix, lipid granules and vesicular bodies being common in the central part are absent in these outgrowths. Plasmodial outgrowths reach the host body surface and adult orthonectids move inside them using their cilia and stopping from time to time. The plasmodial outgrowths penetrate the ciliated epithelium, then males and females leave the host. Duration of emission may vary in different species from 6 to 13 days. The second mechanisms of emission is common for the orthonectid parasites of mollusks. Our observations of Rhopalura philinae from the gastropod Philine scabra lead to the conclusion that males and females leave their host practically simultaneously. When the plasmodium attains the terminal stage of its development most of the host entrails are already displaced by plasmodial mass. It causes breaks in host body walls and hence to emission of sexual individuals. During this process, which lasts about 24 hours, the mollusk dies. The same mechanism was observed in Rhopalura littoralis--parasite of the gastropod Onoba aculeus. Our investigations of emission ways reveal that the plasmodium of orthonectids has a potency of directing growth and can form certain structures. The process of forming the plasmodial outgrowths is coordinated in time and space. These outgrowths have certain directions inside the host body and the maturation of sexual individuals is clear related with the development of plasmodium outgrowth system. Our results suggest that forming of plasmodial outgrowths is an element of development of the united and highly integrated system. It is necessary to emphasize the capability of plasmodium to accomplish such morphogenetic transformations. This fact argues that plasmodium is a part of parasite organism and not host cells modified, like some experts supposed.