L. Forró - É. Murai szerk.: Miscellanea Zoologica Hungarica 5. 1989 (Budapest, 1989)
Sey, O.: Laboratory model for studying experimental fascoilopsiosis
RESULTS AND DISCUSSION Embryonic development The eggs are light yellowish-brown in color. Measurements of eggs are 0. 12-0. 14 mm in length and 0. 07-0. 09 mm in width. Freshly laid eggs usually contain zygote or blastule of a few cells. At first, the development is rather slow but later it becomes more rapid. During incubation at 30°C no significant change can be observed in the first 5-6 days, except for the growth of the embryo and its oval elongation. With the growth of the embryo, the vitelline cells gradually decrease in number and the place of the vitelline cells embracing the embryo is now occupied by big vacuoles. On the 10th and 12th days the embryo reaches a size of 120- 150 ^um and seems to be capable of hatching. In fact, however, hitching begins only later, as the miracidia has become capable of hatching not only morphologically but also physiologically. Hatching of miracidia usually begins on the 13th day and continues on the following one or two days. Hatching is stimulated by artificial illumination. Mi racidia The hatched and free-swimming miracidium is fusiform with an X-shaped eye-spot,, covered with cilia; an animalcule is 150- 190 ,um in length and 40-50 jum in width. The body of miracidia is covered with flat epidermal cells in fiye transverse rows, according to the formula 6:6:6:6:6. The life-span of miracidia kept in tap water at room temperature was 10-12 hours but their infectivity was limited to the first three hours only. Intermediate snail host Several sets of infections were performed with young (2-3 weeks old, 3-4 mm in diameter) snails, S. hemisphaerula, at room temperature (20-22°C). Eight-ten miracidia were added to snails kept in water individually in the hollows of a microtiter plastic plate. The take varied between 2 5 and 46 per cent. A total of 40 snails of the same age were cultivated in an aquarium to detect their egg-producing capacity. They produced an average of 0. 4 egg masses per day. A count of 12 3 egg cluthes showed that the average number of eggs in an egg mass is 11-12. Accordingly, each snail produced 5-6 eggs per day on average. The incubation period of eggs varied from 5 to 7 days, the hatching rate varied from 93 to 100%. They matured in 2 5- 30 days after hatching, when the shell diameter was 3-4 mm. Deposition of the eggs lasted up to three months or so. By this time 80% of the initial sum of snails died. Intramollusoan larval stages After penetration, the invading miracidia metamorphose into sporocyst. They grow rapidly until they reach an average size of 40,0 by 110^um. The rediae, formed within the sporocyst, liberate by their rupture, in the lymphatic spaces. The rediae have a large digestive tract with a pharynx, birth pore and locomotor appendages. Rediae can be fully developed within 32-35 days afte" infection. First cercariae were liberated on the 35th day after infection. Newborn cercariae are poorly developed and migrate slowly to the hepatopancreas of the snail where the growth of their organs continues and they mature eventually. The first cercariae emerged on the 60th day after infection. Metacercariae are whitish in color, vigorously swimming organisms. When they were killed in hot water, their body was somewhat spearshaped or oval, measuring 180-195 by 150-1 58 jjm while the tail was 490-520 by 52-59 jUm. The emergence of cercariae was observed in the area which is contiguous with the anus of the snail. At this point a teat-like appearance can be seen and the cercariae extruded with considerable force during the processe. Emergence of cercariae showed an interesting periodicity. It took place in the evening and night hours (emergence began from 10 p. m. and continued through night up to 6-7 a. m. Almost fifty percent of the cercariae emerged between midnight and 2 a. m. After the emergence, cercariae swim away from the snail and they begin to encyst soon on the surface areas of the container. For the practical purpose, a sheet of cellofane being put on the water surface proved to be the best metacercarial trap. Metacercariae are spherical and have a diameter of 130- 140 v um. The wall of the cyst is thin (7 jim) and thus the metacercariae are less resistant to dryness. Through the cyst-wall the
