What is the relationship of a flatworm and horseshoe crab?

The relationship between a flatworm and a horseshoe crab is one of predator and prey. Flatworms are carnivores and feed on a variety of small invertebrates, including horseshoe crab larvae. Horseshoe crabs are arthropods that are related to spiders and scorpions. They are found in shallow coastal waters around the world. Flatworms are found in a variety of habitats, including marine, freshwater, and terrestrial environments.