Naked axons and symmetrical synapses in coelenterates

Examination of sections of the marginal ganglion of the jellyfish Cyanea and the hydromedusan Phialidium by the electron microscope, in a region where nervous tissue is readily identified on account of its abundance, reveals the following features. Nerve-cell bodies and axons are crowded together without special glial cells. The axons form a layer between the cell-bodies and the mesogloea and the spaces between them are continuous with other intercellular spaces and with the mesogloea. Features typical of nerve-cells in other animals are mitochondria, Golgi region (= γ-cytomembranes), neurotubules (= canaliculi) about 16 mμ wide, and several types of vesicle ranging in size from 50 to 200 mμ, including synaptic vesicles of 50 to 100 mμ. Features not typical of nerve-cells are the modified (possibly sensory) cilia on the dendrites of bipolar cells and the absence of clumps of Nissl substance and neurofilaments. Synapses between axons (or with a perikaryon) have a synaptic cleft of 18 to 22 mμ and a crowded row of synaptic vesicles within the neurones on each side of the synapse.