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.
Fish development
Neurulation, the formation of the central nervous system, is different in fishes than in most other chordates.Convergence and extension in the epiblast recruits presumptive neural cells from the epiblast towards the midline where they form a neural keel. A neural keel is a band of neural precursors that develops a slit like lumen to eventually become the neural tube. The neural tube begins as a solid cord formed from the ectoderm. This cord then sinks into the embryo and becomes hollow, forming the neural tube. This process contrasts with the process in other chordates, which occurs by an infolding of the ectoderm to form a hollow tube.