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

plasm contained no iodophilous vacuole. No projections or membranaceous envelope were found. Tissue sites: Development histozoic (Fig 2B). Developing plasmodia and cysts were found in the secondary lamellae, so that they surrounded the capillary of the lamellae and were located on the membrana basialis of the secondary lamellae, covered by the respiratory epithelium (Fig. 2B). DISCUSSION Only a few of the Myxobolus species listed by Landsberg and Lorn (1991) show definitive morphological characteristics by which they can be unanimously distinguished from other species. The description of a new Myxobolus species should therefore contain, besides the morphological characteristics of the spores, the host specificity and tissue specificity of the given species as an aid to identification. As regards host specificity, some myxosporeans have been recorded from a large number of fish species and e.g. Myxobolus cerebralis has been found to infect almost all salmonids. At the same time, Myxobolus drjagini and M. pavlovskii develop only in the closely related Hypophthalmichthys molitrix and H. nobilis. According to Achmerow (1955), Thelohanellus spp. show an even stricter host specificity, and several Thelohanellus spp. infect only one fish species, the common carp. Therefore, it seems to be obvious that among the species described hitherto there are several synonyms; at the same time, it is also possible that a species known under a common name covers several undescribed species. It seems very unlikely e.g. that M. exiguus, a common parasite of the Mugil and Liza species living in brackish water, could infect freshwater cyprinids, although several papers have reported such infection. Tissue specificity is a neglected but very important point in the identification of a species. Different Myxobolus spp. have a strict affinity to a certain type of host tissue, and there are muscle, nerve, epithelium, cartilage and connective tissue specific species. For instance, M. cyprini develops exclusively in muscle cells (Molnár and Kovács-Gayer, 1985). In view of the above points, in the description of the two species found by us we accepted the following considerations: (1) Barbus spp. inhabiting the water system of River Tigris do not live in other faunal regions; therefore, myxosporeans infecting them are also very likely to represent new, undescribed species. (2) The two species found are typical gill parasites, which means that they can be compared only with gill-dwelling parasites of the known species. Both parasites found by us distinctly differ from the known species by their specific location. M. karuni obviously starts its development in the gill endothelium, while M. persicus forms plasmodia between endothelial and epithelial cells of the secondary lamellae. M. karuni has relatively large spores with elliptical polar capsules. This species distinctly differs from the known species by its polar filaments coiled up 10 to 11 times in the capsule, and by the elongated shape of the intercapsular appendix. The shape of M. persicus spores resembles that of M. oviformis Thelohan, 1892, M. exiguus Thelohan, 1895 and M. lobatus Dogiel, 1934, but only the last mentioned species