Observations on living specimens confirm the hypothesis formulated by Chabaud et al. about traumatic insemination in Oncholaimus oxyuris. A detailed description is given of the copulation, the formation of copulation pores, interstitial channels and the injection of glandular material and sperm. The terminal structures of the demanian system of O. oxyuris are not pre-existing structures, but are built by the combined action of males, ejaculatory and female substances. The injected sperm is temporarily stored around the uvette in the main duct; from there they reach the uterus through the ductus uterinus. With each copulation, a new copulation pore and interstitial channel are formed. After withdrawal of the spicules a ‘wound plug’ seals the pore. When copulations follow at short intervals, sperm of the previous insemination are evacuated to the intestine through the osmosium. The hypothesis is formulated that subsequent copulations result in the fertilization of subsequent egg masses by different males.
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