1. In some hydroids (freshwater coelenterates or invertebrates; such as, jellyfish), a filamentous tactile or touching process: Such palpocils, or sensory filaments, can be detected on sea anemones and corals.
2. A minute soft filamentary process springing from the surface of certain hydroids and sponges: The palpocils are tactile hairs appearing on coelenterates, which are animals with a saclike body with only one opening and tentacles with stinging parts, and occur in medusa and polyp forms.