Sacculina Carcini

a parasitic marine crustacean of the suborder Rhizocephala of the order Cirripedia. The saclike body has rootlike appendages that thread through the body of the crab that the crustacean parasitizes. Four or five days after the larvae, or nauplii, hatch from the eggs, they metamorphose into cypris larvae, which are clad in a bivalve shell, and convert to a parasitic way of life. After attaching itself to a crab, the larva discards its trunk portion, which includes the extremities and the shell. The remaining head portion embeds itself in the crab and metamorphoses into a hermaphroditic adult individual.