555-545 million years ago


Hemichordates evolved a dorsal nerve cord--one of the major characteristics they share with chordates--but primitive nerve plexuses remained the predominant components of the nervous system.

Although hemichordates possess a dorsal nerve cord, the major component of the nervous system in acorn worms is a nerve plexus within the skin. Middorsal and midventral longitudinal cords form in this plexus (Benito, form Harrison 1997; Hickman, p. 699-700; 706). Hemichordates, like echinoderms, possess a primitive nerve net (plexus) similar to that of coelenterates (Hickman, p. 645-6, 699). This primitive nerve net not only exists in the epidermis, but along the gastrointestinal tract as well. Some neurons in the plexuses are not polarized (Beklemishev, vol. 3). In acorn worms (enteropneusts), the visceral and superficial nervous systems are still largely separate. In most enteropneusts the nerve cord is solid but it contains a lumen in some (Beklemishev, vol. 3, p. 134). Genes which higher vertebrates use to determine position along the anterior/posterior axis of the neural tube (such as Pax6, Pax2/5/8, En, Fgf8/17, Otx, Emx, Dlx1/6, Nkx2.1) are also present in lower chordates (Murakami, 2005). Hemichordates utilize ACh and NE as neurontransmitters as do higher vertebrates (Benito, form Harrison 1997, p. 98). The anterior end of hemichordates is sensory and may include chemical-sense receptors (Beklemishev, vol. 3, p. 136).