The Euphorb‘s have rather peculiar flowering structures that produce these rather peculiar seed capsules. The flower is so unusual that botanists had to give it it’s own name – cyathium. I can’t even describe it here, just follow the link if you’re interested. Those white things that look like petals are not. They’re simply referred to as appendages.
In any event, they’re cool looking plants. Poinsettia’s are in the Euphorbia genus. Look closely and you’ll see the same structure.