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plural hemipenes or hemipenises
(zoology) Each of a pair of intromittent organs of male snakes and lizards, usually held inverted within the body but everted for reproduction via erectile tissue. quotations
Charles […] found her stroking the goanna in such a way – no one else could do it – that its pale hemipenes emerged pale and spiky from their sheaths.
1985, Peter Carey, Illywhacker, Faber and Faber, published 2003, page 475