Why Don't Horseshoe Crabs Evolve?

We were talking in class today about how random change is inevitable in a species, and how changes are "selected" by the environment and thus end up being found in future generations. That's well and good, but we have a problem: horseshoe crabs (someone also mentioned alligators in class). Horseshoe crabs have been around for half a billion years, at least, at haven't really changed since them. Why, if random change is inevitable, have certain species (seemed to) have avoided it? 

While browsing this topic on the web, I came across one Yahoo! thread that proposed a possible solution to this wrench in Darwin's argument. "Morey000" said that limited variability of offspring and limited variability "in natural or sexual selection" could have caused the crabs to stay the same. Extending Morey's guess about limited natural selection, if we assumed that the horseshoe crab's environment has remained stable for the last half a billion years, then it would make sense that limited changes in DNA would be selected by the environment (because the horseshoe crabs are doing just fine, thank you, and don't need any changes to survive). But I would guess that over the last half a billion years, the environment has to have changed for these creatures in some way (especially when considering that those years encompassed an ice age). 

So why do some species change when others don't? And what does this mean for Darwin's theory of constant random adjustment to a constantly changing environment? I'm not sure, but the horseshoe crabs still seem to be doing just fine, thank you.