DISTRIBUTION OF METACERCARIAL CYSTS OF THE DIGENEAN CENTROCESTUS FORMOSANUS)NISHIGORI,1924)ON THE GILL FILAMENTS AND GILL RAKERS OF BIG-HEAD CARP, ARISTCHTHYS NOBILIS

The present paper deals with distribution of the metacercarial cysts infrapopulation of Centrocestus formosanus(Digenean: Heterophyidae)in the gill filaments and gill rakers of Aristchthys nobilis from August 1993 to November 1994. The results indicated that most of the matercercarial cysts distributed in the gill filaments from August 1993 to March 1994, and distributed in the gill rakers from April to November 1994. When the body length of the host varied form 30 to 90 mm most of the matercarial cysts distributed in the gill rakers, but distributed in the gill filaments when the body length of the host exceeded 90 mm. The metacercarial cysts percentage of the gill filaments and gill rakers changed with the growth of the host, the percentage of the gill filaments increased with the growth and varied from 0 to 100%,but the gill rakers declined with the growth and varied from 100% to 0. Distribution of the metacercarial cysts in the different groups of gill filaments showed the same regular changes in different months and groups of body length, so did the different groups of the gill rakers. The second groups of gill filament had more matacercarial cysts, then the third, the first and the fourth, but the first groups of gill rakers had more metacercarial cysts and then the second, the third and the fourth. The distribution of the metacercarial cysts in the gill filaments and gill rakers showed aggregated distribution and the degree of aggregation increased with the mean density of the metacercarial cysts infrapopulation.