THE HORSESHOE CRAB

Millions of years before dinosaurs began roaming Earth, strange-looking creatures were scuttling across the seabed. They looked like they were wearing spiked helmets, with little eyes on top and a sharp tail sticking out the back. The horseshoe crab still exists today and belongs to an order of animals known as Xiphosura, from ancient Greek words meaning 'sword' and 'tail'. Despite their name, they're not actually crustaceans, but more closely related to spiders.

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Fossilised horseshoe crabs have been found dating back to the Upper Ordovician, around 450 million years ago. Since that time, their descendants - four living species - have barely changed their appearance, and they're often misleadingly called 'living fossils'.

Despite their ancient origins, horseshoe crabs play an important role in the modern world. Most people have, at some point, crossed paths with a lifesaving dose of the horseshoe crab's vivid blue blood. The blue colour comes from the oxygen-carrying pigment haemocyanin, an equivalent to the red haemoglobin in vertebrate blood.