Bdelloura candida

Bdelloura candida, sometimes called the Limulus leech, is a species of planarian flatworm in the family Bdellouridae. The flatworm has a commensalistic relationship with the Atlantic horseshoe crab, Limulus polyphemus, living in the horseshoe crab's book gills and around joints and legs. The common name Limulus leech is a misnomer, as the species is a flatworm and not a leech, and it is not parasitic, as leeches are.