Blanket octopus:

The males have a specially modified third right arm which stores sperm, known as a hectocotylus. During mating, this arm detaches itself and crawls into the mantle of the female to fertilize her eggs. The male dies shortly after mating. The females carry over 100,000 tiny eggs that are attached to a sausage-shaped calcareous secretion held at the base of the dorsal arms and carried by the female until hatching. Blanket octopuses are immune to the poisonous Portuguese man o’ war, whose tentacles the male and immature females rip off and use for defensive purposes.  

Here’s video:

Via.