• Three species of progenetic trematodes were described to belong to the genus Asymphylodora Looss, 1899 by helminthologists. They are A. progenetica Serkova and Byk-howsky, 1940, A. dollfusi Biguet, Deblock and Capron, 1956, and A. amnicolae Stunkard,1959. In this paper is reported the fourth of such examples of a digenetic trematode,which grows to maturity within a single molluscan host. The gastropod parasitised isStenothyra toucheana Heude, inhabiting the Min River delta and its tributaries. Havingcompared the morphological features of the adult and larval stages with above-men-tioned three species as well as other species of the genus as described by previousworkers, the author considers the present species to be new to science and the name A. stenothyrae sp. nov. is given. The worms are spindle-shaped, wider near the middle and narrower toward bothends. The total body length is 0.715-0.963 mm and body width 0.360-0.457 mm. Theoral sucker is subterminal, 0.072-0.132 mm in diameter. It is followed by the pharynx0.042-0.057 mm in transverse and vertical diameter. The esophagus is of considerablelength. It bifurcates into two intestinal caeca, surrounding the acetabulum and extend-ing posteriorly to about one third of the body length. The acetabulum measures 0.137-0.192 mm in diameter. A single testis is present. It is oval in shape, measuring 0.090-0.154 mm and 0.064-0.094 mm in longitudinal and transverse diameter. Two vas effe-rens originate from its anterior part and run anteriorward to near the posterior edge ofthe acetabulum, where they unite together forming a vasa deferentia entering the cirruspouch. The latter contains a bipartite seminal vesicle and a long cirrus. The ovary ispyriform and curved. The posterior proximal portion of the ovary, which bends an-teriorly giving rise to an oviduct which proceeds forward and dilates to form a seminalreceptacle. It is connected with a Laurer's canal at its basal portion where theoviduct turns towards the ootype. The vitellaria consist of large and round follicles,7-10 in number on each side of the body. The vitelline ducts are thick and conspicuous.A big vitelline reservoir is situated in front of ovary. From the reservoir a generalvitelline duct leads to the oviduct as it comes near the ootype. The uterus begins witha few winding loops behind the acetabulum, extending posteriorly and then anteriorlyoccupying both lateral fields. It finally terminates as the metraterm and opens to thegenital pore situated on lateral side of body. Eggs are oval or pyriform in shape, mea-suring 0.034-0.043 mm in length and 0.015-0.017 mm in width. Those eggs in thedistal portion of uterus are fully embryonated, containing a ciliated miracidium.The larval stages recovered in the present study consist of the redia, the cercaria, metacercarial cyst and the juvenile adult. The redia is elongated in shape with somewhatnarrow anterior end. It possesses no muscular appendages. They measure 0.807-1.384mm in length and 0.269-0.403 mm in transverse diameter. There is a birth pore situat-ed near the pharynx. The pharynx has a diameter 0.047-0.076 mm in length and 0.064-0.080 mm in width. The gut is connected to the pharynx by a short slender neck. It issac-like in form measuring 0.068-0.257mm in length by 0.064-0.269 mm in width.The mature redia contains germ balls in various stages of development. Usually thereis only one mature cercaria which is capable of moving round in the body cavity. The larva of A. stenothyrae is a true cercariaeum having no tail or any vestigialremains of the organ. The body measures 0.58 by 0.30 mm in longitudinal and transversediameter, being oval in shape and cuticle covered with minute spines. The oral sucker issubter minal, 0.10 by 0.11 mm. The acetabulum is slightly larger, measuring 0.14 mm. Thepharynx is small, 0.040 by 0.045 mm, and the esophagus is of considerable length,0.07mm. It bifurcates into two intestinal caeca surrounding the anterior portion ofacetabulum and extend posteriorward. There are 15 pairs of unicellular penetrationglands situated anteriorly to the acetabulum with two bundles of gland-ducts leading for-ward within the dorsal wall of oral sucker and opening on its anterior margin. The ex-cretory bladder is pear-shaped, lined with 4-5 epithelial cells on each side of the vesicle.It receives two collecting tubes connected on its anterio-lateral border. The tubes pro-ceed forward in sinuous course making coils and loops in the lateral fields of the bodynear the penetration glands. After several windings, it descends posteriorly to the mid-level of the acetabulum and divides into anterior and posterior branches. The maturecercariae can disperse from the body of the infected snail. They crawl out and gatheron the tentacles of the host, attaching with their ventral suckers to the tentacle andstretching their bodies on both sides. When two snails meet and contact with tentacles,the worms can pass from one snail to the other. Such behavior of the cercariae hadbeen observed and recorded by Wunder (1924), Wesenberg Lund (1934) for Cercariae-um paludinae-impurae and by Stunkard (1959) for Cercariaeum of A. amnicolae. The metacercarial cysts were found from snails even not infected with rediae. Thecyst has round contour measuring 0.40 mm in diameter. The excysted metacercariais 0.62mm in length and 0.38 mm in width. The oral sucker measures 0.11 by 0.12 mm. Thepharynx is 0.04 by 0.05 mm. By this time the acetabulum is 0.16 by 0.17 mm, and theintestinal caeca are very well developed. The reproductive organs all begin to take theirshapes. Both the encysted and the excysted metacercariae are able to grow to maturityand lay eggs without being taken by vertebrate or invertebrate hosts, but since thereexists the metacercarial cyst stage, some fishes may play the role of facultative hosts forthe parasite and this has not been discovered. The phenomenon of progenesis has attracted the attention of helminthologists sincethe time of von Siebold (1887) who reported the maturity of a trematode-species in thebody of Astacus astacus. Giard (1887) first created the term 'Progenese', indicating thegrowth to maturity of reproductive organs in juvenile animals. Dollfus (1924) used thisterm to denote such phenomenon in trematodes and conceived its significance. Fromthat time on various authors reported on the subject. Helminthologists like Joyeux(1923), Mathias (1924), Dollfus (1927, 1932), Joyeux, Noyer and Baer (1932), Alice Buttner. (1950), McIntosh (1935) and Stunkard (1959) etc., not only gave new eviden- ces, but also discussed on the nature and origin of this phenomenon. In the present study, basing on the compilations of Alice Buttner (1950) a new listwas made on the known records of progenesis. Additions were made from various sourcesincluding those species reported from China (Wu, 1937, 1938; Komiya and Tajimi,1941; Yeh and Wu, 1950; Yeh and Wu, 1955; Tang and Tang, 1963). Altogether 34species of progenetic trematodes were recorded either from molluscs, crustaceans or in-sects. They belong to 23 genera, and 17 families. Incomplete as it may be, the list in-dicated that progenesis must have been a common occurrence among trematodes in thepast. At the present it is probably the reversal to the ancient growth patterns.
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