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Synapses provide pathways of communication between cells throughout the animal kingdom wherever a nervous system is present. The most primitive or elementary nervous system that can be readily recognized is the nerve net of coelenterates . The first synapses described at the ultrastructural level in nerve net animals were symmetrical synapses in marginal ganglia of jellyfish (Horridge and Mackay, 1962) . Later, two types of synapses were reported (Jha and Mackie, 1967) in a hydromedusan, Sarsia : (a) symmetrical synapses with vesicles on both sides of a specialized membrane and (b) asymmetrical synapses with vesicles on only one side of the synaptic junction. Asymmetrical or morphologically polarized synapses have now been demonstrated ultrastructurally in a sea pen (Buisson and Franc, 1969 ; Buisson, 1970), a primitive hydromedusan (Westfall, 1970 a), a sea anemone (Westfall, 1970 b), and various other species of Cnidaria (Westfall et al ., 1970) . The ultrastructural confirmation of neurons in Hydra by Lentz and Barrnett (1965) excited a number of investigators to the possibility of demonstrating synaptic contacts in this elementary nervous system. To date, there have been no reports of morphologically specialized contacts between cells of Hydra that might indicate the precise region of chemical mediation of an impulse (see Lentz, 1968) . In the present study ultrastructural evidence is provided for the presence of polarized interneuronal synapses and neuromuscular and neuronematocyte junctions in two species of Hydra .

ULTRASTRUCTURAL EVIDENCE OF POLARIZED SYNAPSES IN THE NERVE NET OF HYDRA