Arms and suckers from an O. insularis embryo. This is a Naef stage XVIII embryo of O. insularis embedded in agarose and observed using DIC microscopy. Details of one arm and two suckers can be observed at the cellular level; the focal point was switched often to capture details at different depths. During the fast movement of sucker suction, a group of cells surrounding the central part was observed to be drawn towards the central pocket only to return to their original position thereafter. This action is also shown in slow motion. This cellular displacement, clearly governed by muscular contraction, could be related to sucker suction, even during developmental stages.