See also: prostomium. 3. (Arthropoda: Insecta) Membranous tissue around the base of the mouth. 4. (Bryozoa) Modifications of the area around the orifice. 5. (Echinodermata: Ophiuroidea) Membranous area surrounding the mouth; on the aboral surface of the jaw. 6. (Mollusca: Gastropoda) Thickened rim or lip around the mouth; the margin of the aperture.