Dr. Murai Éva szerk.: Parasitologia Hungarica 16. (Budapest, 1983)

M. intermedius (Hunter, 193U) M. microphagus Ingles, 1936 M. rankini Bravo-Hollis, 1941 Megalodiscus ferrissianus Smith, 1953 is a nomen nudum. Diagnosis. Opisthodiscinae. Body conical or pyriform. Pharynx Megalodiscus-type, primary pharyngeal sacs well developed. Oesophagus with bulb, caeca straight, extending acetabulum zone. Two testes oblique, in anterior intercaecal fields. Cirrus pouch small, genital opening immediatelly postbifurcal, Subclavatus-type. Ovary: small, posterior position of intestine, curved inward posteriorly. Egg: large, thin-shelled, sometimes embryonated. Excretory pore: middorsal, anterior to acetabulum. Acetabulum ventroterminal with central prominence. Parasitic in rectum of amphibians and rarely snake. Type species: Megalodiscus americanus Chandler, 1923 (Fig. 27) Key to species of Megalodiscus 1. Caeca extending beyong ovary, near to anterior margin of acetabulum 2 - Caeca extending posterior end of ovary M. rankini 2. Vitellaria spreading from posterior end of posterior testis to anterior margin of acetabulum 3 - Vitellaria extending from anterior margin of anterior testis to posterior of ovary M. temperalus 3. Vitellaria constituting single group each side 4 - Vitellaria forming two groups in each side M. intermedius 4. Diameter of ventral sucker larger than that of body M. americanus - Diameter of ventral sucker smaller than that of body M. microphagus ZOOGEOGRAPHICAL CONSIDERATIONS Amphistomes are one of the few groups of digenetic trematodes which are parasites of all classes of vertebrates from the teleosten fishes to mammals. As the amphibians are intermediate in the sequence of vertebrate taxa, the study of phylogenetic relationships of their amphistomes promises will aid us in recognizing of trends in the evolutionary process of the whole group of amphistomes. It is generally held that the digeneans of vertebrates evolved from the ancestral rhabdocoel turbellarian stock which had existed in the pre-vertebrate geological times. It is supposed that vertebrates of different classes, which have appeared in subsequent ages, were colonised by the flukes derived from these ancient stocks and they tried to make contact with all vertebrates existing in a given period depending on the capacity of the ancient amphistomelike stocks. As there was a significant degree of time displacement in the sequence of appearance of various vertebrates, a more or less rigid specificity evolved to the definitive hosts living in a given time before another (phylogenetically younger) vertebrate-group whould have appeared on the scene. This explains why vertebrates evoluing later on picked up their amphistome parasites not from among those adapted to the formerly evolved definitive hosts, but from the ancient amphistome-like stock. Amphistomes, designated to the family Diplodiscidae in the sense of the present review, are predominantly parasites of amphibians, except a few, e.g. P. cornu (parasite of fresh water fish) and D. trifoliata (parasite of fresh water turtle). Both of them, however, are much more