isthmus
From ZooTerms (Dictionary of Invertebrate Zoology)
isthmus (noun; Latin from Greek isthmos, neck, narrow place): (Mollusca: Bivalvia) Part of the mantle that secretes the horny uncalcified material (conchiolin) of the ligament. 2. (NEMATA) The middle part of a muscular esophagus, often constricted; a narrow section of the esophagus.
