<P> In Cnidaria larvae, neurons are not distributed homogenously along the anterior - posterior axis; Cnidaria demonstrate anatomical polarities during the differentiation of a nervous system . There are two main hypotheses that attempt to explain neuronal cell differentiation . The zootype hypothesis says that regulatory genes define an anterior - posterior axis and the urbilateria hypothesis says that genes specify a dorsal - ventral axis . Experiments suggest that developmental neurogenesis is controlled along the anterior - posterior axis . The mechanism by which this occurs is similar to that concerning the anterior to posterior patterning of the central nervous systems in bilaterians . The conservation of the development of neuronal tissue along the anterior - posterior axis provides insight into the evolutionary divergence of coelenterates and bilaterians . </P> <P> Neurogenesis occurs in Cnidaria not only during developmental stages, but also in adults . Hydra, a genus belonging to Cnidaria, is used as a model organism to study nerve nets . In the body column of Hydra, there is continuous division of epithelial cells occurring while the size of the Hydra remains constant . The movement of individual neurons is coupled to the movement of epithelial cells . Experiments have provided evidence that once neurons are differentiated, epithelial cell division drives their insertion into the nerve net . As neurogenesis occurs, a density gradient of neuronal cells appears in the body . The nerve net of each cnidarian species has a unique composition and the distribution of neurons throughout the body occurs by a density gradient along the proximal - distal axis . The density gradient goes from high to low from the proximal to the distal end of the Hydra . The highest concentration of neurons is in the basal disk and the lowest (if neurons are even present) is in the tentacles . During development of Hydra, the amount of neurons gradually increases to a certain level, and this density is maintained for the duration of the organism's life - span, even following an amputation event . After amputation, regeneration occurs and the neuron density gradient is reestablished along the Hydra . </P> <P> A nerve net is a diffuse network of cells that can congregate to form ganglia in some organisms, but does not constitute a brain . In terms of studying nerve nets, Hydra are an ideal class of Cnidaria to research and on which to run tests . Reasons why they are popular model organisms include the following: their nerve nets have a simple pattern to follow, they have a high rate of regeneration, and they are easy to manipulate in experimental procedures . </P> <P> There are two categories of nerve cells that are found in the nerve nets of Hydra: ganglion and sensory . While ganglion cells are normally found near the basal ends of the epithelial cells, sensory cells generally extend in an apical direction from the muscle processes of the basal ends . While Ganglia generally provide intermediary connections between different neurological structures within a nervous system, sensory cells serve in detecting different stimuli which could include light, sound, touch or temperature . </P>

The nervous system in cniderian consists of nerve nets