"Cnidaria have no brains or even central nervous systems. Instead they have decentralized nerve nets consisting of : sensory neurons that generate signals in response to various types of stimulus, such as "smells"; motor neurons that tell muscles to contract; all connected by "cobwebs" of intermediate neurons. As well as forming the "signal cables", intermediate neurons also form ganglia that act as local coordination centers. The cilia of the cnidocytes detect physical contact, and nerves inform cnidocytes when prey or attackers are "smelt" and when neighbouring cnidocytes fire. Most of the communications between nerve cells are via chemical synapses, small gaps across which chemicals flow. As this process is too slow to ensure that the muscles round the rim of a medusa's bell contract simultaneously in swimming the neurons which control this communicate by much faster electrical signals across gap junctions"
Nervous systems of cnidarians (PubMed)
"Cnidarians have simple nervous systems and it was probably within this group or a closely-related ancestor that nervous systems first evolved.
The basic plan of the cnidarian nervous system is that of a nerve net which, at some locations, has condensed to form nerve plexuses, or circular or longitudinal nerve tracts which may be syncytia.
At the ultrastructural level, many cnidarian neurons have the combined characteristics of sensory, motor, inter- and neurosecretory neurons and thus appear to be multifunctional. We propose that these multifunctional neurons resemble the ancestors of the more specialized neurons that we find in higher animals today."
The primitive nervous system of cnidarians is strongly peptidergic, indicating that evolutionarily "old" nervous systems use peptides http://en.wikipedia.org/wiki/Peptide as transmitters.
Cnidarians and the evolutionary origin of the nervous system (PubMed)
The peptidergic nervous system of coelenterates (PubMed)