Evidence is presented that eggs and sperm of Spirorbis borealis are shed through the nephridioducts, and that fertilization occurs externally to the body but inside the tube.
The eggs fail to develop in vitro unless sperm is present. In individuals collected in winter and maintained at 15 or 19° C, the male and female gonads mature more or less simultaneously. Among groups of individuals sperm is usually shed first and soon afterwards a large proportion may be observed to have extruded ova.
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