Xiphosurida horseshoe crabs are one of the most ancient arthropods on Earth that have survived to the present day. All their relatives (scorpions, ticks, haymakers) settled on land more than 400 million years ago, and only horseshoe crabs remained faithful to the marine environment.
They are so ancient that using the example of these arthropods, they study the behavior of trilobites, which completely died out a quarter of a billion years ago: how they shed, changing the old skin, or escaped from their pursuers.
Horseshoe crabs breathe through their gills, but they can also move on land, since the special structure of the gills allows them to absorb oxygen from the air while the gills retain moisture.
The horseshoe crab's blood is not red, but blue because of the high content of copper in it.
