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Production of the cercariae of Centrocestus formosanus and Haplorchis pumilio was not affected by a 10-day starvation of the snail host Melanoides tuberculata, and there was no circadian rhythm in the pattern of cercarial emergence. Daily average cercarial productions from each snail were 1,643 for C. formosanus and 689 for H. pumilio at 25 C and 500 lux illumination. The number of cercariae was moderately correlated with the size of snail host. The heaviest sheddings ever observed from 1 snail in 24 hr were 63,400 cercariae for C. formosanus and 3,470 cercariae for H. pumilio. The cercariae of C. formosanus could emerge in the dark, but the number was much less than in the light. In contrast, the cercariae of H. pumilio emerged equally well with or without light. Within the temperature range tested, the emergence of cercariae occurred at 15-35 C, but not at 10 C. Shedding of C. formosanus cercariae increased with the rise in ambient temperature, whereas the greatest shedding of H. pumilio cercariae occurred at 25 C. Life-span of the cercariae was temperature-dependent. The most favorable temperature for survival of C. formosanus was 15 C, at which some survived for 160 hr, and 20 C for H. pumilio, with the longest survival of 130 hr.

Miracidial penetration and intramolluscan development of a zoophilic strain of Schistosoma japonicum were studied in the highly susceptible Oncomelania hupensis chiui and a highly resistant population of Oncomelania hupensis formosana. When exposed to 20 miracidia, an average of 3.6 miracidia penetrated O. h. chiui and 3.4 penetrated O. h. formosana. The distribution of sporocysts in the 2 snail subspecies was similar: 70.4% migrated to the cephalopedal sinus, 14.2% to the anterior viscera, and 3.1-4.9% to the posterior viscera, tentacle, mantle collar, and foot. In O. h. chiui only 1 out of 118 sporocysts degenerated and there was no host cellular response. In O. h. formosana, all the sporocysts survived for 12 hr, but by 5 days postinfection (DPI) all were dead and by 10 DPI they had disintegrated completely; the encapsulation was encountered in only 3 out of 78 sporocysts or 3 out of 23 snails, suggesting that it played a minor role in the defense against S. japonicum. All of the O. h. formosana snails of 10 and 15 DPI showed pronounced leukocytosis and elevated phagocytosis in the heart and increased cell debris in tissue spaces, which coincided with the disappearance of the parasites. It is suggested that these phenomena were provoked in response to degradation products released by the dead parasites.

Eggs of Echinostoma macrorchis were laid at the 1-cell stage and took 9 days (26-27 C) or 6 days (30 C) to hatch. The following accounts are based on studies undertaken at 24-26 C. Maximum survival of miracidia was 7 hr. Miracidia reached the ventricle of Gyraulus chinensis 15-20 hr after penetration and transformed into sporocysts. The earliest degeneration of sporocysts occurred 20 days postinfection (DPI), but some survived for 120 days. Mature mother rediae were first seen inside the sporocyst at 7 DPI, then were released to the ventricular cavity and migrated to other anatomical locations of the host. Among the germ balls present in a sporocyst, 1-3 developed concurrently to the mother redia stage. Young daughter rediae first appeared at 15 DPI and mature ones at 19 DPI. Snails of larger size produced more daughter rediae and cercariae than smaller ones. Initial shedding of cercariae from infected snails occurred between 25 and 48 DPI, with a mean of 34 DPI. The cercariae were phototactic and each snail shed a daily average of 34 cercariae. Gyraulus chinensis was proven to be the only snail capable of serving as the first intermediate host. Tadpoles of Rana latouchi and Bufo bufo, 9 species of gastropods, the bivalve Corbicula fluminea, and I unidentified planarian species served as experimental second intermediate hosts. Echinostoma macrorchis from Taiwan differed from those of Japan in several respects: metacercariae of the Taiwanese parasite were smaller but were already infective 12 hr after encystment, and they matured into adults in 7 days in rats, whereas metacercariae of the Japanese parasite were larger, requiring 35 hr to become infective after encystment, and took 13 days to grow into adults.

Six new areas for Oncomelania hupensis were found on Taiwan: three in Nan-tou, and 1 each in Chia-i, Hua-lien and Tai-tung counties. Most of the new habitats, unlike previously known areas, are not in alluvial plains, but are at higher elevations. The Nan-shan-chi colony in Nan-tou county is located at the altitude of 740 m, which is the highest of all known Oncomelania localities on the island. Oncomelania has now been found in 9 out of 15 counties on Taiwan. Shell features of the newly-discovered snails resemble those of Oncomelania hupensis formosana but further study will be required for final subspecies designation. Nearly 60,000 oncomelanids from both old and new localities were examined for schistosome infection with negative results.