Primitive Nervous System - Nerve Net
A nervous system creates its appearance for the first time in the phylogeny among Cnidaria. In this group the nerve cells make an irregular nerve net or plexus. In hydras we have a simple condition. The nerve net is build up of bipolar or tri-polar and rarely multi-polar neurons. The sensory nerve cells are distributed in between the epithelial cells; they send their nerve fibers to the sub-epithelial nerve plexus. The plexus is located beneath the epidermis. In hydroid polyps the plexus is concentrated around the mouth, suggesting the beginning of a centralised nervous system. Several cnidarians have more complicated nerve nets; for example they may have as well as gastro dermal nerve net within the gastrodermis in addition to the epidermal nerve net like in sea anemones. A double nerve net in the same body layer is also common in some coelenterates. There is quite a lot of autonomy, in that, isolated parts of the animal behave as if parts of the body. There is no central controlling mechanism apparently because there is no distinct central nervous system. It is as well poor in specific responses to particular stimuli. Transmission is un-polarised, in that impulse can be transmitted in either direction along the synapse not like in higher animals. This occurs because both terminals secrete transmitter substance. So the nerve plexus in this group shows diffuse, un-polarized transmission, autonomy of parts and scarcity of reflexes.