Ultrastructural evidence for the existence of two types of neurosecretory cells in the abdominal ganglia of the chelicerate arthropod, Limulus polyphemus.

Evidence suggesting the existence of two types of neurosecretory cells in each abdominal ganglion of Limulus polyphemus has been obtained by light and electron microscopy. After Helly fixation the two cell types are readily distinguished from other neurons by the Azan method, but they react weakly when stained by paraldehyde fuchsin. Type I cells are larger, more regular in shape, and found more anteriorly in each ganglion. They contain apparently cylindrical secretory granules, many dictyosomes, and numerous cytoplasmic vesicles. Type II cells produce spherical granules, contain fewer dictyosomes, have less conspicuous cytoplasmic vesiculation and possess more prominent parallel arrays of rough endoplasmic reticulum. Granules similar to those found in both cell types are present in the neuropile and certain nerves, but the specific pathways of the axons of these cells have not yet been determined.