630-600 million years ago

worm nervous system

Higher flatworms evolved a cerebral ganglion/brain, nerve cords which stretched down the longitudinal axis of the body, glia, rod-like bipolar photoreceptors, and the genes, receptors, and neuron cell types which are found in higher animals.

Higher flatworms possessed the first central nervous systems. In addition to a primitive nerve net, they possess longitudinal nerve cords and a cerebral ganglion ("brain"), although it was not well developed. The brain developed around a statocyst, homologous to the statocyst of ctenophores. These first "brains" developed at the end of the animal which was becoming specialized to deal with sensory information. This brain had little connection with or control over the diffuse nerve net throughout the rest of the body (Beklemishev, vol. 2, p. 50-1; Raikova, 2000). There were cranial nerves which consisted of short pairs of nerves which stretch from the brain to the pharynx and regions of the head (Rieger, from Harrison, 1991).

Flatworm nervous systems possessed a number of features known in higher animals. Unipolar, bipolar, and multipolar neurons are known in the most primitive flatworms, the acoels (Rieger, from Harrison, 1991). Higher flatworms possessed multipolar neurons with dendritic spines and soma exterior to axons, electrical synapses, tetrodotoxin sensitive sodium channels, voltage-gated fast sodium channels, dark adaptation of the eye, habituation, cells similar to hypothalamic neurosecretory cells, and the use of NE, E, serotonin, ACh, and neuropeptides in signaling (Sarnat, 1985) Nitric oxide and nitric oxide synthase are known in flatworms and higher animals but not in cnidarians (Tandon, 2001).

Of 116 genes known to be involved in the development of the brain and nervous system of flatworms, more than 95% were shared with higher bilateran animals such as nematodes, flies, and humans. Homologs of all 116 existed in humans. Homologs of about a third of these genes existed in organisms which lack a nervous system, such as yeast (Mineta, 2003). These shared genes include FGF, noggin, frizzled (a Wnt receptor), immunoglobulin/cadherin family members, otx, neuropeptide Y, NCAM, BMP receptors, and rhodopsin (Mineta, 2003). Members of the Wnt gene family are involved in the formation of the vertebrate brain and also in the regeneration of planarian brain (Marshal, 2003). Serotonin, enkelphans, and endorphins are known in flatworms (Rieger, from Harrison, 1991).

Glia similar to astrocytes exist in planarians (Sarnat, 1985) and some neurons are wrapped in a glial sheath (Rieger, from Harrison, 1991). The genes atonal1 and NP1 are expressed only in bilateran neural tissue but are expressed in both neural and muscle tissue in cnidarians, supporting a common origin for neurons and muscle cells (Seipel, 2004; Muller, 2003).

The simplest eyes in flatworms are pigmented spots of epithelial cells. Other flatworms possess sunken cups of cells. In turbellarian flatworms, there are bipolar receptor cells (as in vertebrates) whose distal ends are rod-like. These eyes cannot produce a visual image, merely distinguish between light and dark (Barrington).