Dr. Murai Éva szerk.: Parasitologia Hungarica 21. (Budapest, 1988)

morphological features at hand. In this context amphistomes are characterized by the following traits. Adults are amphistome type or monostome evolving from amphistome plan; usually conical, with strongly muscular body; hermaphroditic; with strongly developed postero-terminally located acetabulum. Oral opening surrounded by well-developed and muscular pharynx, oesophagus may be muscular (with sphincter, muscular thickening or bulb) or without it. Caeca usually straight, rarely undulating. Lymphatic system present; excretory system etenostome, excretory bladder saccular, two main descending trunks open to bladder. Parasitic in each class of vertebrates. Metacercariae usually encyst freely; cercariae large, amphistome-type with unforked tail. Body either with dense pigmentation (Cercaria pigmentata) or with light pigmentation (Cer- caria intermedia) or without pigmentation (Cercaria diplocotylea)

. A pair of eyespots and a lot of cystogen cells present (except Heronimus)

. Protonéphridial system well-developed, excretory bladder non-epithelial; new-born cercariae poorly developed. Rediae and sporocysts rarely absent; rediae without collar. Intermediate hosts usually pulmonate freshwater snails. Miracidium with 6:8:4:2 and 6:6:4:2 epidermal cell formula and without pigment spot. Eggs: large in size, egg-shell thin (freshwater developed) or thick (marine developed); viviparity, ovoviviparity, ovoparity present. Life-cycle dixen type, with one intermediate host. Summarizing, the main, in-group character states of the amphistomes are the posteroterminally located ventral sucker in adult or cercarial stage and the epidermal cell formula of miracidia. Analysing the character states of the trematodes, now regarded to be the monophyletic group of amphistomes, three equally high level taxa can be differentiated: Heronimata Skrjabin et Schultz, 1937; Zonocotylata n. suborder and Paramphistomata Szidat, 1936. Heronimata is a monotypic taxon, its taxonomic ránk is determined by monostome type, and by its special morphological structure of the adults, and by the ancient type of reproduction. The taxon Zonocotylata is a reduced one, including one genus and its species. It has morphological characters of its own (reproductive system, fixative apparatus, etc. ) which justify its taxonomic position in the systematics of the Amphistomida in the present sense. The most successful evolutionary line of the amphistomes seems to be the taxon Paramphistomata whose representatives can be found in each higher taxa of the vertebrate definitive hosts. This group includes the typical forms of amphistomes, species comprising, the amphistomes in a narrow sense (sensu stricto). Several schemes have been proposed in the history of the amphistome systematics, and the amphistomes represented either by familial rank (FISCHOEDER, 1903; STUNKARD, 1925; FUHRMANN, 1938; N'A S MARK, 1937), superfamilial rank (STILES and GOLDBERGER, 1910; MAPLESTONE, 1923; TRAVASSOS, 1934; SOUTHWELL and KIRSHNER, 1937; BAER and JOYEUX, 1961; YAMAGUTI, 1971), subordinal rank (SZIDAT, 1936; LA RUE, 1957; SKRJABIN, 1949) or ordinal rank (TRAVASSOS et al. 1969; ODENING, 1974; BROOKS et al. 1985). In the taxonomic structure of the above-mentioned schemes, the involvement of monostome groups emerges several times even in that of the latest one (BROOKS et al. 1985). As it was previously indicated, it is advisable to exclude monostomes in question from the scope of amphistomes which preserve the monophyly of the group until the structure of their larval stage does not support their actual taxonomic position. Hence, the order-level diagnosis of Amphistomida agrees well with ODENING'

s (1974) description, supplemented with the newer information referring to the taxa now attached to Amphistomida. The new systematic structure proposed by the present writer (see later) can be best compared with SKRJABIN's (1949) and YAMAGUTI'

s (1971) schemes which are the most comprehensive ones and the latter is a relatively recent classification. Both of these systematics, however, were based on morphological features of the adults and little attention was paid to the weighted characters of the ontogenetic cycle, bearing phylogenetic information. The concept of phylogenetic systematics, formulated by HENNIG (1966) renders classification