660-630 million years ago

cnidarian with eyes

Ancestral cnidarians evolved nerve cells which their descendants incorporated into complex nervous systems. It seems that the enteric plexus of neurons aroudn the gastrointestinal tract of vertebrates is equivalent to the plexuses which surrounded the gastrovascular cavities of these cnidarians. Ancestral cnidarians were the first animals to possess eyes with a homolog of rhodopsin to detect light.

Cnidarians possess two types of nervous cell: sensory and deeper ganglion cells. Ganglion cells synapse with each other and with muscle cells. The neurosensory cells seem to be the more primitive of the two since they are less differentiated (they lack dendrites, for example) and they are very abundant in lower vertebrates. Examples of neurosensory cells in higher animals include the receptor cells of parietal and lateral eyes (including rods and cones), infundibular cells in fish, and olfactory neurons. In humans, only the receptor cells of the olfactory epithelium and retina are of this type. In cnidarians, ganglion cells possess both Nissl substance and dendrites. These neurons are considered as primitive since they lack myelin and impulses are transmitted slowly. Only in bilateran animals do neurons transmit impulses in only one direction and possess both dendrites and axons (Ariens). Some hydra neurons are wrapped in sheaths similar to glia (Mackie, 1990).
Nerve cells in cnidarians are not part of a centralized nervous system, they form a diffuse nerve network which may have ganglia (Fretter, p. 68). Certain regions of the body may have a greater concentration of neurons than others (Fretter). In cnidarians, there are actually two nerve nets; one is more involved in feeding and the other in moving. The first nerve net allows local responses while the second activates all muscles simultaneously (Fretter). These two nets are usually interconnected, but in some species there is little or no connection. In some species, neurosensory cells of the epithelia synapse with the nerve net (Hickman). The giant fibers linking the senses to the muscles are similar to the giant fibers of many invertebrates (Fretter, p. 68). Ganglia can control the regular rhythms of movement in jellyfish and comb jellies (through a structure known as the apical organ; Fretter, 68 & 94).

The nervous plexuses of the submucosa and muscularis layers of the gastrointestinal tract of vertebrates (referred to as the enteric nervous system) consist of a diffuse nerve net which is largely independent of the central nervous system. This nerve net is similar to the nerve net which surrounds the gastrovascular cavity of cnidarians and evolved prior to the brain and nerve cords of bilateran animals. The connections between the enteric nervous system and the central nervous system evolved gradually in vertebrates. In fish, there is no vagal stimulation of the GI tract beyond the stomach. (Stevens,p. 273).

Cnidarians such as jellyfish are the simplest animals to have eyes. Jellyfish have a receptor which is similar to vertebrate RXR (very similar to rhodopsin) which binds retinoic acid and then binds the DNA of crystallin genes, just as in both vertebrates and invertebrates (Kostrouch, 1998).