Each individual of a mating pair tries to shoot a dart into the body wall of the other by forceful eversion of its containing dart sac. The dart is coated with mucus from a pair of 4-branched mucus glands opening close to the opening of the dart sac. The penis of each is everted, like a sock turned inside out, through the atrium and out of the genital pore by hydrostatic pressure of haemolymph (blood).
1 dart sac.
2 basal stem of mucus gland near to dart sac.
3: inverted grey penis that is everted for copulation.
4: genital atrium.
5: genital pore; opening to outside of animal for entire reproductive system.
6: vas deferens which brings sperm to the penis.
7: white penial flagellum that forms tail of spermatophore.
8: epiphallus that forms body of spermatophore.
9: white retractor muscle (severed) which, after copulation, inverts the penis like a sock pulled outside in, as in this image.

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