Morphological substrates of conduction in the colonial hydroid Hydractinia echinata. 1. An ectodermal nerve net.

Abstract

AbstractCorrelative light and electron microscope observations suggest that the colonial hydroid, Hydractinia echinata, possesses a single ectodermal nerve net in gastrozooid and dactylozooid polyps, and in the uppermost ectodermal sheet of the basal mat, but not in free stolon tips. Two distinct cell types comprise this nerve net — ganglion cells and sensory cells, the neurites of which characteristically possess microtubules and electron dense, membrane‐bound vesicles. The sensory cell body possesses an apical ciliary apparatus which distinguishes it from the ganglion cell type. Ultrastructural features of ganglion and sensory cells are presented in relationship to their distribution. Gastrozooids and the basal mat contain ganglion and sensory cells; dactylozooids contain ganglion cells primarily on the concave side of the polyp. Significantly, distinct bundles comprised of several neurites are found directly above the epithelio‐muscular cell processes of the gastrozooid and dactylozooid. Although synaptic junctions have not been found between ectodermal nerve cells and the muscle processes, they have been found bridging two nerve cells.