How do I pass an argument to the index function?

If I have

Code:

class Admin extends MY_Controller {
     public function index($task = NULL)
     {
       

     }
}

And a hyperlink that goes to base_url('admin/open/');
It will look for a method or function called open in my Admin controller. 
What if I want it to pass a url segement called open to the Index function? What do I do?