20. Chordate neural evolution.

Chordates are animals named after a stiffening rod (the notochord) that appears in their bodies at some time during development.

Chordate neural evolution may be schematised as shown. Looking at the left diagram first -- the central nervous system arises as a groove along the back of the organism, which then sinks into the body as in 1. Along most of the the groove the lips come together to form a tube as in 2 but in part of its length it remains open so that it looks V-shaped as in 3.

Different parts of the groove develop different functions. The main division is into Input or Afferent pathways marked A and Output or Efferent pathways marked E.

As shown in the right diagram, the Input and Output parts of the groove gradually evolve into more and more complex versions of four main types of circuit -- Visceral, Branchial, Somatic and Special Somatic.

The evolution of mammalian (including human) branchial circuits has been emphasised with a yellow border because this circuitry is homologous with the demonstration worm taste circuit that we looked at earlier. We shall look at our human version of the worm taste circuit in the next picture.