The fine structure of the hypostome and mouth of hydra

The hypostome and mouth of fresh-water Hydra were examined by scanning electron microscopy. The external surface of the hypostome possesses cnidocils, possibly sensory hairs, and small spiny protrusions surrounding the mouth; the internal surface has cylindrical microvilli, free flagella and adherent flagella. The adherent flagella are most numerous close to the mouth where they cause the cell surface to appear smooth when viewed at low magnifications. Free flagella and leaf-like microvilli increase in prominence towards the tentacles and enter on proper. The edge of the mouth has an abrupt boundary marking the apposition of epidermal and gastrodermal cells. A transitional groove occurs at the boundary and the cells underlying the groove are smaller than those on other regions of the hypostome. The transition groove may represent a site of cell loss in normal cell turnover. Some of the small underlying cells may represent nervous elements involved in regulating hypostome activity during the feeding reation.