Yet Another Mystery Theme V: The Mystery Theme Strikes Back by VaccaD

Question 3

WHAT ARTHROPODS are members of the order Xiphosura and (despite their name) most closely related to spiders? They have nine eyes (some of which can see into the ultraviolet spectrum) and their blue copper-based blood contains amebocytes, a white blood cell analogue harvested for use in the LAL test for detecting bacterial endotoxins.