The sensory border of the land planarian Bipalium kewense (Tricladida, Terricola)

The lunular headplate of Bipalium kewense is limited all around its margin by a distinct sensory organ, consisting of a row of papillae, intercalated with ciliated pits. We have investigated the detailed morphology of this sensory region by means of scanning and transmission electron microscopy, as well as, immunohistochemistry. Epithelial cells of the papillae have insunk nuclei and a microvillar border. The dense cytoplasm contains a system of smooth, elongate, vesicles. Monociliated ends of multipolar dendrites come to the surface of the headplate through the epithelial cells. Pear-shaped ciliated pits insert among the papillae. These pits are also provided with sensory dendrites having longer cilia, that are interpreted as chemo-receptors. The distal ends of receptors at both locations are provided with belt-ne septate junctions, especially evident following lanthanum. impregnation. Immunohistochemical experiments by indirect immunofluorescence have localized substance P in the ciliated receptors, thereby confirming their sensory nature.