660-630 million years ago
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).
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).