I hadn't until today, sadly enough. At least, not that I recall.
They are also refered to inaccurately as tadpole shrimp or sheild shrimp. "True" shrimp are in a different (and less-primitive) crustactean group.
Apparently, they are easy raised as pets, with eggs living dormant for an extended time, making them available to children as "instant pets"
They remind me of a horseshoe crab, and like the horseshoe crab, are decidedly a "living fossil"
In the sense that I am surprised to be surprised by an object of natural bewonderment, this reminds me of a solifugid, aka the wind scorpion, which I just had no idea about at all and as such came as a huge surprise to me.
This is all embarrassing to say. But each time I learn something, I am catching up ;-)