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It would seem far more natural to derive the vertebrate from some free swimming form like the schematic worm, whose largest nerve-cord lay on the dorsal surface because its branches ran to heavy muscles much used in swimming. Later the other nerve-cords degenerated, for such a degeneration of nerve-cords is not at all impossible or improbable.
This is an elastic rod of cartilage, lying just beneath the spinal marrow or nerve-cord, which runs backward from the brain. The nerve-centres are therefore here all dorsal, and the notochord or skeleton lies between these and the digestive or alimentary canal. The skeleton of the clam or snail is purely protective and a hindrance to locomotion.
My best guess would be that it is simply a thickened portion of the upper median surface of the intestine to keep the "balls" of digesting nutriment or other hard particles in the intestine from "grinding" against the nerve-cord as they are crowded along in the process of digestion. Once started its elasticity would be a great aid in swimming.
It is the largest nerve-cord in the body, running down the middle of the back of the thigh to supply the muscles of two-thirds of the leg.
Those given off from the small of the back just above the hips also run together to form, first a network and then a big single nerve-cord, called the sciatic nerve, which many of you have probably heard of from the frightfully painful disease due to an inflammation of it, called sciatica.
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