Horse shoe crabs have stayed the same for 400 million years. Is evolution broken or something?

This is a common sentiment that people specify to discredit evolution. But it doesn't.
(I'm assuming you're right about horse shoe crabs, to convey my point, I know nothing about them )

Why have horse shoe crabs not evolved in any noticeable manner for 400 million years?
Because they don't have to! Evolution by natural selection essentially dictates that organisms adapt to their surrounding environment, through genetic processes such as mutations, so as to successfully survive and reproduce from generation to generation.

If horse shoe crabs are perfectly adapted to their environment already, it ensures that these successful genes will be passed down to future generations of horse shoe crabs. If no major ecological or environmental disruption causes them to adapt, then these genes will remain similar until one does (In this case, 400 million years).

All that does is confirm evolution is working as they have adapted to the environemt well and the need for change has been minimal.

Also, you can't use one species to discredit what has been proven amoung millions of others, it doesn't work thats simply I'm afraid.

 So, since the Earth and it's environment hasn't changed in 400M years, horseshoe crabs don't need to evolve?

The earth and the environment have, it's that have not needed to change because they have always been well adapted to the environments they have lived in for the past 400M years I would imagine.

If that one case disproves evolution in your eyes perhaps I should give a case of extreme rapid evolution. (this is paraphrased from an issue of new scientist) 10 years ago scientists put acidophobic bacteria into an acidic environment, over 40,000 generations they became acidophiles and needed the acid which used to kill them to stay alive... that is a 100% evolution, they are now a completely new species. That's ten year to do a complete evolution.

Exactly. If it ain't broke, why fix it?