The male argonaut is a fraction of the female's size. He fills a specialised arm, the hectocotylus, with sperm, detaches it, and it swims away under its own power to find her and deposit its contents. He does not go with it.
When 19th-century naturalists found these arms inside female argonauts, they assumed they were parasitic worms and gave them a genus name — Hectocotylus. Georges Cuvier named it in 1829. It took until the 1850s for anyone to work out that the worm was a male octopus's arm.
