Sacculina is a genus of barnacles that parasitize crabs. The female Sacculina larva finds a crab and walks on it until it finds a joint. It then molts, injecting its soft body into the crab while its shell falls off. The Sacculina grows in the crab, emerging as a sac, known as an externa, on the underside of the crab’s rear thorax, where the crab’s eggs would be incubated.
Winchell, Alexander Sketches of Creation (New York, NY: Harper & Brothers, 1870)