Nerve elements in the planula of the hydrozoan Pennaria tiarella

The nerve elements described by light microscopy for the hydrozoan planula have not previously been identified ultrastructurally. This electron microscopic study confirms the presence of two distinct nerve cell types in the planula of the hydroid Pennaria tiarella. Type I nerve cells occur at the base of the ectodermal epithelium just apical to the forming foot processes of the epitheliomuscle cells. The perikaryon contains mitochondria, microtubules, neurosecretory granules, and a prominent Golgi body. Neurites rich in microtubules project from these cells and form a nerve plexus of transversely and longitudinally oriented processes throughout the length of the planula. The Type II nerve cell extends from the free surface of the planula to the mesoglea and bears a single cilium surrounded by long microvilli. The Type I and II nerve cells closely resemble the sensory-motor-interneurons and neurosensory cells of Hydra.