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.