Cross connections between opposite arms and legs give us automatic, alternating walking movements. Watch a cat, horse or dog walk. Invertebrate Nervous Systems - solid nerve cords Sponges don't have a nervous system. Coelenterates have a nerve net similar to what we have in our intestines. It elongates the body tube and shortens it and extends and withdraws tentacles.
Simple flatworms have a bilateral ventral nerve cord with a ganglia repeated down the nerve cord.
The brain is a close connection of ganglia in the head region. Crayfish also have a bilateral, ventral nerve cord in which the two cords are closer together. There is a ganglion in each segment and the brain is a collection of closely spaced ganglia. The human brain starts as a flat plate of cells the curl up into a tube with a hollow in the center. It grows and matures from head to tail.