Argonautidae are actually the egg cases / craddles of pelagic octopus like cephalopods. Only the female makes the egg cases, and the male is very tiny, so irrelevent in fact that they were not discovered until the late 19th century! The males have a special arm called the hectocotylus, which they use to transfer sperm to the female. The arm detaches during the process.

Read more and see live pictures via the link below:-

Scientists solve millennia-old mystery about the argonaut octopus