The neuronal basis of pharyngeal ingestion and swallowing can be conveniently studied in the marine mollusc Navanax inermis because of the small number of neurons involved in the behavior, reproducible identifiability of many individual neurons and simple muscle arrangement. The Navanax pharynx contains 3 approximately orthogonal muscle groups, circumferential, longitudinal and radial, arranged in well defined layers. Pharyngeal peristalsis involves sequential circumferential constriction, apparently with coordinated local pharyngeal expansion produced by radial muscle. Circumferential and longitudinal muscles consist of discrete, well defined bands. The number, position and arrangement of circumferential and longitudinal bands show little interanimal variability; these bands are individually identified by sequential number. Regional morphologic specializations of circumferential bands presumably facilitate peristalsis. Circumferential and longitudinal bands form a two dimensional reference system defining position on the pharyngeal surface. In the accompanying paper circumferential motor fields are described in terms of identified motoneurons innervating identified bands, and radial motor fields are located with respect to the coordinate system of overlying longitudinal and circumferential bands.