Model/Controller: get id of current item and use it in controller

Question

I got a view in my component's frontend, which catches data from model using the ID of the element:

protected function populateState($ordering = null, $direction =  null)
{
    $app        =   JFactory::getApplication();
    $id         =   $app->input->get('id', null, 'int');            
    $this->setState('id', $id);

    parent::populateState();        
}   

/**
 * getItem function.
 * 
 * @access public
 * @param mixed $pk (default: null)
 * @return void
 */
public function getItem($pk = null)
{
    $pk = (!empty($pk)) ? $pk : (int) $this->getState('id');
    ...
    return $this->_item[$pk];
}

Okay, this works fine, the model gets the data and the view is showing it.

Now I would like to create a form which saves the input of a user and attach it to the id.

So I created a form like this

<form id="save" class="form-validate" action="<?php echo JRoute::_('index.php?option=com_bestia'); ?>" method="post" >

... 
<button type="submit" class="btn validate"><?php echo JText::_('COM_BESTIA_SUBMIT', true); ?>
<?php echo JHtml::_( 'form.token' ); ?>
<input type="hidden" name="task" value="item.save" />
</form>

I also created a hidden-field to add the id to the post-data

<field  name        =       "id" 
        type        =       "hidden" /> 

But in my controller I can not access the id like this:

public function save($key = NULL, $urlVar = NULL)
{
    JSession::checkToken() or jexit(JText::_('JINVALID_TOKEN'));
        $jinput         =       JFactory::getApplication()->input;
        $data           =       $jinput->get('jform', array(), 'post', 'array');
}

How can I submit the id to my controller?