not use circles, so we must use a square instead. As a kanji element, it can refer to any opening - the mouth of a jar, or the mouth of a river, or the entrance to a cave, etc.
and bottom parts are white, and the middle is the colored iris. As an element, this kanji may appear as-is on the left, or rotated on top: 罒. You may wish to remember the alternate version as look down on.