In most species and hybrids, the nectaries are tubular. They may be colored, but they are small and almost hidden. Double flowers occur when the nectaries extend and become flattened, looking like sepals. As you can see, this extension of the nectaries takes different forms. These still show a bit of their normal tubular shape - quite nifty, I think.
Doubles can vary in form, too.
This pink, fading with age to green, reminds me of my rugosa roses a bit. For some reason, this also reminds me of apple blossoms. Although Graham says the anemone flowered forms make better garden plants, because the flowers don't get as battered by the UK's winter rains, I'd still love to have this in my garden. He did say, that our drier winters, this side of the pond, might make a difference. Graham says that if you cross a double with a single flowered hellebore you will get a percentage of anemone flowered plants.