How did horseshoe crabs end up with such special blood

How did horseshoe crabs end up with such special blood? Like other invertebrates, the horseshoe crab has an open circulatory
system. This means their blood isn't contained in blood vessels, like ours.Instead, horseshoe crab blood flows freely through
the body cavity and comes in direct contact with tissues. If bacteria enters their blood, it can quickly spread over a large
area. Pair this vulnerability with the horseshoe crab's bacteria-filled ocean and shoreline habitats, and it's easy to see
why they need such a sensitive immune response.