The Crab Armer is a horseshoe crab. It only appears in Super Darius II game.
Things start off rather sedately with the little horseshoe crab thingies Crab Armer fires off at ground level. After a few shots, though, he warms up and gets somewhat hasty. He fires off five spinning (and not destructible, sadly) blades matching the spikes on his dorsal armour and then rushes you, tail rampant. Oh, and he fires a few heat-seekers in the process. Now, when the rush is over he beats the ground with his tail and stirs up lumps of rock and fires more missiles at the same time, too. I'm not quite sure if his tail fires some lasers too, as a final attack in the cycle. You might want to check it out to be sure.