Some older species Like the horseshoe crab are still alive. The blood coagulates and for this, they are mercilesly hunted down and milked or drained completely. The blood is also made with copper as a base material. Fascinating creatures.

EDIT: Okay I thought I made things clear and easy enough to understand. Horseshoe crabs have very sensitive blood that clots up (coagulates) when it comes in contact with bacteria. Their blood is based on copper (Hemocyanin), unlike ours which is based on Hemoglobin or iron. Having the blood based on copper and clogging it up makes the blood very anti bacterial and is one of the earliest types of an immune system we know predating dinosaurs by a long shot. They are mercilessly hunted down to test vaccines since their blood clogs up in presence of bacteria but they are also eaten or used as fish bait.