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Ronald W. Davies, Trefor B. Reynoldson, A Comparison of the Life-Cycle of Helobdella stagnalis (Linn. 1758) (Hirudinoidea) in Two Different Geographical Areas in Canada, Journal of Animal Ecology, Vol. 45, No. 2 (Jun., 1976), pp. 457-470
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Davies et al. (1976) studied this question.
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