Upside down jellyfish

Upside-down jellyfish are awesome. I mean, regular jellyfish are pretty cool looking, but these guys take the cake. And not the upside-down pineapple cake, but the upside-down jellyfish cake. What are they called? Cassiopea, similar to the constellation although to be honest I'm not sure if it has an extra i in the pea or not. The internet is giving me mixed results. They do excrete stinging cells, so if you get too close you might not be very happy. Just admire from afar, and you're all good.