This picture here shows the crab itself:
Now this is interesting for its own sake. but look at this image:
What's interesting is how the horesehoe crab hasn't changed over so long a time. We are trained by textbooks, by Darwin himself (for those who actually read him), and of course by the media, to think of evolution as constantly happening in a relentless rat-race of competition, always needing to adapt to the changing environment or improved competitors.
But here is an organism that has been running the race by staying in place. In fact, at the DNA level the currently available data are scant, but this group of arthropods has continued to evolve, so that its DNA sequences reflect very distant common ancestry--for some groups that look like horsehoe crabs and the horseshoes themselves, hundreds of millions of years. That means that the accumulation of mutations in their DNA has continued to evolve more or less as would be expected.
Now much of the DNA that is used as a molecular clock to estimate species ancestral split times has little if any function and is chosen for study specifically for that reason, since it is more reliably clock-like than functional DNA that can be affected by quirky natural selection.
But if adaptation is so intense a pressure to change in response to one's surroundings, how is it that there has been essentially no evolution of functional aspects of the genome in 150 million years? The simple answer is "Well, no problem! Their environment hasn't changed." There's no easy way to refute that, though geological and paleoecological data might suggest otherwise. Environment is not just the temperature and salinity of the ocean, it includes networks of plants and animals, each preying on the other and so on. So it is perplexing that so little change has happened.
Perhaps horseshoe crab physiology is different enough to show selective pressures, and it's just the shell that hasn't changed. But the fossil anatomy is not just 'skin' (shell) deep, but is in so many of the details. Even just by chance one would expect that minor variations in the shape and shell structures etc. of these creatures would have arisen. Really, one should expect more than trivial changes.
There should be many ways to modify shape and stay 'fit'. Basic body plans are rather stable, but they do change, as we see by the variation among, say, fish and humans or wasps and butterflies. One might say that the crabs' shape is determined by just one or two genes that controls so many of the conserved parts. Then, any mutation might be harmful and removed.
But developmentally, such traits and complexity rarely, if ever, is so simple and usually involves many different genes' contributions. So how such conservation has happened--and there are other examples such as flies or ants entombed in millions-years old amber etc., who look just like our friends and pests today--is rather mysterious, and very interesting to try to explain in terms of our current knowledge of developmental and population genetics.
So, eons later, we have the last walk of this poor doomed crab. But although it was going somewhere as its spirit left its shell for the Other World, its lineage wasn't going anywhere at all.