Zinnias

I'm having to water them sometimes twice a day so they survive the heat, but four of them are blooming with more to come. Three pink and one scrawny orange one. The largest one looks like it's sprouting an arm, which I didn't think they did. I'm also wondering if I dead head the blooms once they're spent, whether I can get more blooms out of it, or whether each one is a one trick pony? The bloom on the first one that opened has been there about four days now and shows no signs of fading.