Since then the superior and inferior and the right and left are linked to one another by the same common source (by which I mean that which controls their movement), and further, everything which is going to make a movement in each such part properly must have the original cause of all the said movements arranged in a certain definite position relatively to the distances from it of the said sources (and these sources in the individual parts are in pairs arranged co ordinately or diagonally, and the common centre is the source from which the animal's movements of right and left, and similarly of superior and inferior, start), and since each animal must have this source at a point where it is similarly related to each of the sources in the four parts described it is clear then how locomotion belongs to those animals only which make their changes of place by means of two or four Reading staseos, delon hoti.