Random fun fact: If you've ever been given some form of intravenous drug, odds are good these guys helped you. Horseshoe crabs have no immune system as mammals register it; their blood just sort of glomps onto whatever enters a wound and that's how they form a scab. Any bacteria or viruses that get into their bloodstream just get swallowed up and either broken down or transformed into scab.
This makes it brilliant for testing if a solution preparation of some drug or another is clean of any foreign bodies. Just squirt a little horseshoe crab blood in a sample of the solution and bam. If it forms globs, it's tainted. You failed. If not, try again.
Horseshoe crabs are amazing little mofos.