The nervous system of parasitic cnidarian Polypodium hydriforme

Abstract

The nervous system of intracellular parasitic cnidarian Polypodium hydriforme at various stages of its life cycle has been studied by the immunocytochemical method using antibodies to FMRF-amide and by electron microscopy. Neurosecretory, sensory, and ganglion cells have been identified both at the parasitic stage (planula and stolon stages, when body layers are inverted) and in free-living animals. These cells are characterized by the presence of round neurosecretory granules about 80–120 nm in diameter. Gap junctions have been detected between nerve cells. Most of the neurosecretory and sensory cells have been observed in the epidermis of sensory tentacles of free-living animals. Sensory cells possess immobile flagella. The chains of ganglion cells are located under the epidermis and penetrate mesoglea. A centriole encircled by a fragment of nuclear envelope, which is a marker of ectodermal lineage cells in Polypodium, has been described in the cytoplasm of the sensory cells, thus proving the ectodermal nature of the nervous system. Like in most cnidarians, the nervous system of Polypodium hydriforme is a network containing FMRF-amide-like neuropeptides. Neither sense organs, nor ring-shaped nerve concentrations have been observed.