Cnidarians, or Pennaria tiarella, are deemedto be the first multicellular organisms in which a nervous system evolved. The origin of the various types of nerve cells in the cnidarians, namely, neurosensory, ganglionic, and neurosecretory, has been described in detail for Hydra. It is believed that all nerve cells arise from an embryonic reserve cell, the interstitial cell. The origin of nerve cells in cnidarians other than Hydra has not been reported. A recent ultrastructurala nalysis of the nerve elements of the planula of the marine hydrozoan Pennaria tiarella identified two types of nerve cells. Type I nerve cells are at the base of the ectodermal epithelium just apical to the forming foot processes of the epitheliomuscle cells. Neurites projecting from this type I nerve cells form a nerve plexus of transversely and longitudinally oriented processes.
Cnidaria have no brains or even central nervous systems; instead, they have decentralized nerve nets consisting of (1) sensory neurons that generate signals in response to various types of stimulus, such as odors; (2) motor neurons that tell muscles to contract; (3) all connected by "cobwebs" of intermediate neurons. As well as forming the "signal cables", intermediate neurons form ganglia that works as local coordination centers. The cilia of the cnidocytes detect physical contact. Nerves inform cnidocytes when odors from prey or attackers are detected 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.
Cnidaria have no brains or even central nervous systems; instead, they have decentralized nerve nets consisting of (1) sensory neurons that generate signals in response to various types of stimulus, such as odors; (2) motor neurons that tell muscles to contract; (3) all connected by "cobwebs" of intermediate neurons. As well as forming the "signal cables", intermediate neurons form ganglia that works as local coordination centers. The cilia of the cnidocytes detect physical contact. Nerves inform cnidocytes when odors from prey or attackers are detected 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.