542-488 million years ago
In urochordates, the brain and nerve cord became more prominent, although the central nervous system supplemented the ancestral nerve nets rather than replaced them. The brain included possible homologs of vertebrate cranial nerves and glial cells.
Urochordates retain a plexus of neurons around gut. (Burighel, from Harrison, 1997, p. 274). Nerve nets represent a major (if not the only) aspect of the nervous system in cnidarians, ctenophores, echinoderms, enteropneusts, ascidians, and some mollusks (Prosser, 1973, p. 644). Although the central nervous system became increasingly more important in higher animals, there is evidence that the ancestral nerve net was retained around the digestive tract. All higher invertebrates and vertebrates possess a nerve net (or more than one) which governs the secretion and muscular contractions of the digestive tract (Hoar, 1983, p. 450). Even in mammals such as ourselves, the primary innervation of the GI tract is a primitive nerve net which makes most decisions independent of input from the central nervous system. The human enteric nervous system (the diffuse plexuses of the muscularis and submucosal layers of the gastrointestinal tract) actually possess more neurons than the spinal cord (Romer p. 547-8).
Larval urochordates possess a dorsal nerve cord and a group of cranial
nerves (2 to 5) proceeding from the brain (Beklemishev, vol. 2, p. 137-8;
Hickman, 716). A vesicle in the tunicate brain may be homologous to the
saccus vasculosus present in the brains of many fish (Svane, 1982). There
is no trace of a telencephalon or an olfactory region (Nieuwenhuys, 2002).
Of the 335 cells in the tuncate brain, 70% are a group of glial cells
called ependymal cells (Nieuwenhuys, 2002). Tunicates possess glia-like
cells which support neurons. (Ruppert, from Harrison, 1997, p. 470). Of
the roughly 330 cells of the tunicate brain, 70% are glial cells (Nieuwenhuys,
2002). Like other lower chordates, there is no myelin. Ciliated ependymal
cells exist along the neural canal, as in vertebrates (Meinertzhagen,