Read Url-Parameters in the Controllers on_start() Methode

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I have a single page, that takes one url parameter. So the urls look like this:
domain.com/[singlepagename]/[id]

and the controller has a corresponding view function:
function view($id) { ... }

now i need access to the id in the on_start() function. but "function on_start($id) { ... }" throws an error and "$this->get()" returns an empty array.

any idea how i can get url parts in on_start()?