ABSTRACT
The phylum Porifera, or sponges, are very simple multicellular parasites. They do not possess a nervous system and it is controversial as to whether they have any neurons. With the evolution of the coelenterates, such as hydra, jellyfish and corals, the identification of nerve cells and muscles is unequivocal. A very large increase in the complexity of the nervous system occurs with the appearance of the Platyhelminthes or flatworms. The grouping of neurons into a head ganglion that provides an integrating center for the nervous system reaches its most complex level of evolution in the brain of Homo sapiens, which receives sensory information from nerve receptors in different areas of the skin called dermatomes as well as from the distance receptors such as the eyes, ears and nose. In the most primitive animals a nervous system consists of sensory neurons bringing in information to an integrating center consisting of a large number of interconnected neurons constituting a head ganglion.
