Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 27. (Budapest, 1994)

same type, and the Plasmodium continues its development in that capsule formed by the host cells. This is how Myxobolus cerebralis develops in the cartilaginous tissue of the head (Halliday 1973), Thelohanellus nikolskii in a location surrounded by the perichondrial cells of the fin rays (Molnár 1982), Myxidium giardi and Henneguya psorospermica in the endothelium of capillaries (Dyková et al. 1987, Dyková and Lom 1978), and Myxobolus kotlani and Thelohanellus hovorkai in the connective tissue (Molnár et al. 1986, Molnár and Kovács-Gayer 1986a). As a result of the parasite's perfect mimicry, the cells attacked by the parasite nourish the developing plasmodium and protect it against external influences, and a host response against the parasite is mounted only in the period of spore formation (Molnár 1982, Dyková and Lom 1988). A typical instance of mimicry and of the protection conferred by host cells is the development of Thelohanellus nikolskii whose large plasmodia could easily fall out of the thin-walled fins if they were not protected by another connective tissue layer formed around the cartilaginous capsule. The organ specificity of myxosporeans The species developing in the muscle cells, endothelium and loose connective tissue may occur at different sites of the body, in various organs: therefore, in their case no organ specificity can be spoken of. The skeletal muscle parasites (e.g. Myxobolus cyprini, M. musculi and M. pseudodispar) can be found in all parts of the fish body (Molnár and Kovács-Gayer 1985, Baska 1987). The endothelial parasite Myxidium giardi may also occur in different organs such as the gills, the liver and the kidney. A species occurring in such variable locations may easily be mistaken for a new species even by specialists (Hine 1975, Komourdjian et al. 1977), especially if the morphology of its spores also shows substantial variability. At the same time, for species parasitizing the epithelium and the cartilaginous tissue, as well as for some myxosporeans showing an affinity for connective tissue an organ specificity is clearly demonstrable. Organ specificity is also determined by the tissue specificity of the given myxosporean. In such cases, the parasite can develop exclusively in a distinct type of epithelial, cartilaginous or connective tissue even within the basic tissue type. An example of such organ specificity is Myxobolus pavlovskii which forms cysts exclusively in the stratified epithelium among the secondary gill lamellae, or Thelohanellus nikolskii which starts to develop in the perichondrial cells of the fin rays. Two parasites characterized by an affinity for connective tissue form cysts on the gills of the common carp. Of them, Myxobolus dispar parasitizes the compact connective tissue of the gill filaments and possibly that of the tunica adventitia of the blood vessels, while M. basilamellaris forms its cysts partly at the base of the gill filaments, partly within the gill arch. Types of the intrapiscine development of myxosporeans The intrapiscine development of myxosporeans is highly diversified. The works of Shulman (1966), Mitchell (1977), Uspenskaya (1984), Lorn and Dyková (1986,