Estimation of cercariae liberated

Estimations of the number of cercariae liberated from infected oysters were made during June and July, 1964. The procedure was as follows: 11 infected oysters were set up in separate bowls each with one litre of sea-water. Bowls were examined daily. The oyster was first removed from the bowl, the water agitated vigorously to disperse the cercariae and then allowed to settle. The total number of cercariae in the field of view of a Zeiss "Opton" microscope using a ×10 eyepiece and 1.6 objective was counted and averaged over 10 counts. This gave the average number of cercariae in roughly 5cc of water. This method gave a means of repeatable, relative approximations. When very few cercariae were present they were counted individually.