630-600 million years ago


Higher worms evolved larger brains which utilized a variety of neuropeptides.

Even in invertebrates, brains became more complex after the most primitive groups. After the flatworms, nervous systems display a greater cephalization and centralization, a deeper migration of nervous tissue into the body away from epithelia, and a greater complexity of behavior is possible. Outside the brain, nervous tissue becomes more centralized because axons between ganglion cells are shortened while axons to sensory receptors and muscles are lengthened (Beklemishev, vol. 2, p. 83) In nemertine worms, the brain is composed of two pairs of ganglia (dorsal and ventral), 2 nerve chords, and smaller nerves. There are many soma in the nerve chords but typically no ganglionic expansions. In the most primitive nemertines, nervous tissue is still subepidermal (Hickman). The two nerve cords of flatworms are homologous to the equal halves of spinal cord (Sarnat, 1985).

In the nematode C. elegans, about 130 putative receptors for neuropeptides have been identified. Some of the neuropeptides involved have not yet been found in vertebrates. In mammals neuropeptides function in a variety of neural pathways, including those involving feeding and sleep. (Nathoo, 2001).