Nervous system
Nervous systems are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.
Aglantha digitale
"Aglantha digitale" is unique among known jellyfish in having giant axons in the subumbrella (the concave inner surface of the bell) which are involved in a rapid escape response. Normally the hydrozoan swims by slowly pulsating its bell, movements produced by muscle contractions which eject water through the velar opening. However, when the animal is stimulated by manipulating the bell margin, or by squeezing or tugging a marginal tentacle, it responds very rapidly, producing up to three vigorous contractions that propel it a distance of about five body lengths. In its pelagic habitat, this hydrozoan finds itself among relatively fast moving crustaceans, fish larvae and arrow worms in a jostling diurnal vertical migration, and the escape response may help it to avoid damage to its fragile tissues. Observations from submersible craft have shown the hydrozoan behaving in this way.