From ZooTerms (Dictionary of Invertebrate Zoology)
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peristome (noun; Greek peri, around; stoma, mouth): 1. The region surrounding the mouth.

2. (Annelida) Segment modified to form part of the head and surround the mouth; buccal segments

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.