Naked axons and symmetrical synapses in an elementary nervous system

THE animals of the phylum Coelenterata, which includes the anemones, corals and jellyfish, are the simplest animals with undoubted nervous tissue, which is morphologically similar to that of higher animals1, and which has all-or-none impulses accompanied by a typical action potential2. The nervous system is peculiar in being arranged mainly as a network, but in medusæ some neurones form strands and ganglia where sensory cells and many fibres are distinguishable3. The usual synapses are between axons4. No structure comparable to a nerve sheath and no nuclei attributable to glia cells have ever been found. In large axons of jellyfish substantial neurofibrillæ fill the axon in life and can be dissected from it5.