Fig. 1. Schematic representation of the four potential mouth-opening mechanisms (A–D) in Clarias gariepinus. The functional elements of the cranial system are represented by different colours: lower jaw, red; hyoid, green; suspensorium, yellow; operculum, blue; cleithrum, yellow and neurocranium, grey. Dark and light colours represent elements that are involved and not involved, respectively, in a particular mechanism. Broken lines correspond to positions of cranial elements and four-bar chains after activity of the mechanisms. More detailed representations of the four-bar systems are given on the right, in which the fixed link is represented by short perpendicular lines. l-an-ch, ligamentum angulo-cerotohyale; l-an-iop, ligamentum angulo-interoperculare; m-l-op, musculus levator operculi; m-pr-h, musculus protractor hyoidei; o-iop, os interoperculare.