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Larval morphogenesis and infection rates of Cercaria pectinata Huet were investigated in regards to histological changes and mortality of the hard clam, Meretrix lusoria $(R\ddot{o}ding)$, in Buan located on the Western Coast of Korea. The trichocerous cercariae, Cercaria pectinata, and i...

Larval morphogenesis and infection rates of Cercaria pectinata Huet were investigated in regards to histological changes and mortality of the hard clam, Meretrix lusoria $(R\ddot{o}ding)$, in Buan located on the Western Coast of Korea. The trichocerous cercariae, Cercaria pectinata, and its sporocysts parasitize mainly in the tissues of goand, digestive gland and gills of the hard clams. One branched sporocyst grows into a great number of cylindrical sporocysts, and a heavily infected clam possesses 2,000 to 5,000 cylindrical spo ro cysts. Each of them produces approximately 30 to 70 germinal cells and cercariae. Of 2,639 clams examined 347 individuals $(13.2\%)$ were found to be infected. The infection rates showed seasonal variations, the rate being highest $(29.6\%)$ in July, and lowest $(2.0\%)$ in March. Whereas, the infection rates did not change significantly with the changes of size class of the shell. No infection was observed in Meretrix petachialis (Lamarck) collected from Myeong-ji, Kimhae Gun located near Busan. In the iefected clams, the gametogenesis (oogenesis and apermatogenesis) did not undergo completely, therefore the heavily infected clam seemed to be castrated. Also the tissues of the digestive gland and gills were compressed by the multiplication of the parasites. When they were kept in aquariums in the laboratory, high mortality has occurred in the infected clams. All of the 60 infected clams died within 35 days.