Fatal error: Class 'Controller' not found

[eluser]Unknown[/eluser]
Hi all
Im new to codeigniter,i build alot of controllers that extends Controller,and all works fine,expect the one that i request using ajax.
so i try to call this class as:

var url=base_url+"system/application/controllers/employees.php?do=show";//no problem with finding this employee class
ajax=getAjax();//return ajax according to browser
ajax.onreadystatechange=showChanged;
ajax.open("GET",url,true);
ajax.send();

but i get this error:
Fatal error: Class 'Controller' not found in C:\Program Files\wamp\www\employees\system\application\controllers\employees.php on line 6


Any help please ?

[eluser]danmontgomery[/eluser]
You shouldn't be calling the controller file directly, you should be calling it the way you would call any other controller

/controller/method/parameters