[help]How can I read the file in StreamingAssets on iPad/iPhone platform???

Hi, When I want to read the local config file, I put the 'a.txt' under 'Assets/StreamingAssets', and read it by WWW class. And the code as follow, WWW www = new WWW("file://" + Application.StreamingAssets + "/a.txt"); This code runs well on PC, Mac Book Pro. But it's wrong on iPhone/iPad. Why ? & How can I read the file under the StreamingAssets folder on iPhone/iPad platform by WWW class? Thank you.