Horseshoe crabs are marine and brackish water arthropods of the order Xiphosura — a slowly evolving, conservative taxa.
Much like (slow) Water Striders (
Aquarius remigis), (relatively sluggish) Coelacanth (
Latimeria chalumnae) and (the current winner on truly slow evolution) Elephant Sharks (
Callorhinchus milii), these fellows have a long history in the fossil record with very few anatomical changes.