Yes I'm a biologist indeed.

Well thats a simple and complicated question at the same time. Note that I could go on several lines discussing this, you have been warned:
I think most people knows what a trilobite is? Those crab like creatures from the cambrian, that crawled the oceans in ages past.
Well those little fellows produced several lines of evolution, most died out, but one still remains, the chelicerata, a taxonomic group that gathers spiders, scorpions, acari, horseshoe crabs, and other creepy things that lurk in the dark

Even little whale "crabs" (in the nasty private zones vermin sense)
Horseshoe crabs are like a walking fossil. They are supposed to be very similar to those first ancestors. From that line evolved crustaceans and arachnids. Arachnids have diversificated a lot since their arrival to the earth, you have things like a scorpion, a web spider, a tarantula the size of a dinner dish that eats birds, and acari that can make galleries and tunnels into your skin. A wonderful group of animals.