** Some say that calling them a "living fossil" is a misnomer, as the species in the fossil record are not the same as the modern Limulus polyphemus (which only dates back 20 million years or so). So be it. To me the term implies something that is strange to our eyes, because of how little it has changed, rather than something as weird as an orchid, weird because of the lengths to which is has changed....so yes, I think it is ok to call horseshoe crabs living fossils, even though, they aren't exactly perfectly unchanged from the Paleozoic era (see the last reference for more info).
Great videos on this site:
Sample of the recent articles that refute the "living fossil" label http://phenomena.nationalgeographic.com/2011/11/22/in-evolutions-race-horseshoe-crabs-took-a-slower-pace/