M. depressor mandibule (Md.) attaches to the ventro-frontal end of the o.ceratohyale (Oc.)
and the ventro-posterior end of the o.hypohyale (Ohyp.) and advancing along the ventral of the
Od., it attaches the other end to the anterior end of the Od. . It shows ventrally a paired long
ellipsoid shape. Contraction of this muscle induces the Od. to let down, then, the opening of the
mouth occurrs