For animals which make the aforesaid change by the help of organized parts (I mean feet for example, or wings or similar organs) have the left and right distinguished in greater detail, while those which are not differentiated into such parts, but make the differentiation in the body itself and so progress, like some footless animals (for example snakes and caterpillars, and besides what men call earth worms), all these have the distinction spoken of, although it is not made so manifest to us.