<P> Developmental neurogenesis of nerve nets is conserved between phyla and has been mainly studied in cnidaria, especially in the model organism Hydra . The following discusses the development of the nerve net in Cnidaria, but the same mechanism for the differentiation of nervous tissue is seen in Ctenophora and Echinodermata . </P> <P> Cnidaria develop from two layers of tissue, the ectoderm and the endoderm, and are thus termed diploblasts . The ectoderm and the endoderm are separated by an extra-cellular matrix layer called the mesoglea . Cnidaria begin to differentiate their nervous systems in the late gastrula . In Hydrozoa and Anthozoa, interstitial stem cells from the endoderm generate neuroblasts and nematoblasts which migrate to the ectoderm and provide for the formation of the nervous system along the anterior - posterior axis . Non-hydrozoa lack interstitial stem cells, and the neurons arise from epithelial cells, which are most likely differentiated from the ectoderm as occurs in vertebrates . Differentiation occurs near the aboral pore and this is where most neurons remain . </P> <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>

The central nervous system is lacking in animals that have radial symmetry