def get_content_descendants_by_type(self, content_id, child_type, expand=None, start=None, limit=None,
                                        callback=None):
        """
        Returns the direct descendants of a piece of Content, limited to a single descendant type.

        The {@link ContentType}(s) of the descendants returned is specified by the "type" path parameter in the request.

        Currently the only supported descendants are comment descendants of non-comment Content.
        :param content_id (string): A string containing the id of the content to retrieve descendants for
        :param child_type (string): A {@link ContentType} to filter descendants on.
        :param expand (string): OPTIONAL: A comma separated list of properties to expand on the descendants.
                                Default: Empty
        :param start (int): OPTIONAL: The index of the first item within the result set that should be returned.
                            Default: 0.
        :param limit (int): OPTIONAL: How many items should be returned after the start index.
                            Default: 25 or site limit.
        :param callback: OPTIONAL: The callback to execute on the resulting data, before the method returns.
                         Default: None (no callback, raw data returned).
        :return: The JSON data returned from the content/{id}/descendant/{type} endpoint, or the results of the
                 callback. Will raise requests.HTTPError on bad input, potentially.
        """
        params = {}
        if expand:
            params["expand"] = expand
        if start is not None:
            params["start"] = int(start)
        if limit is not None:
            params["limit"] = int(limit)
        return self._service_get_request("rest/api/content/{id}/descendant/{type}"
                                         "".format(id=content_id, type=child_type), params=params, callback=callback)