Definitely are arachnids and they do have 8 legs, as do ticks and mites who're also in the same class. It's thought that scorpions did evolve in the oceans (they do look similar to lobsters if you think about it  

 ) and are Arthrapods like them and crabs and, back in the fossil records, trilobites. They all have jointed legs in common with spiders who're obviously the most well-known archnids  :ugeek: