Horseshoe crabs have been around for over 450 million years, and they have survived five mass extinctions.
They are often referred to as "living fossils" because they have changed very little over time.
Horseshoe crabs are also used in medical research.
Their blood contains a material known as Limulus amebocyte lysate (LAL).
It is used to test for bacterial contamination in vaccines and other medical goods.
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