Nerve net
In the Xenacoelomorpha, a phylum of bilaterally symmetrical animals, members of the subphylum Xenoturbellida also possess a nerve net.
The emergence of true nervous tissue was once thought to have followed the divergence of last common ancestor of Porifera (sponges) and Cnidaria and Ctenophora.
Although Porifera do not form synapses and myofibrils which allow for neuromuscular transmission, they do differentiate a proto-neuronal system and contain homologs of several genes found in Cnidaria which are important in nerve formation.
[8] Sponge larvae differentiate sensory cells which respond to stimuli including light, gravity, and water movement, all of which increase the fitness of the organism.
[10] Nerve nets are found in species in the phyla Cnidaria (e.g. scyphozoa, box jellyfish, and sea anemones), Ctenophora, and Echinodermata.
Coelenterates diverged 570 million years ago, prior to the Cambrian explosion, and they are the first two phyla to possess nervous systems which differentiate during development and communicate by synaptic conduction.
[9] Although animals with nerve nets lack a true brain, they have the ability to display complex movements and behaviors.
The presence of a nerve net allows an organism belonging to the aforementioned phyla of Cnidaria, Ctenophora, and Echinodermata to have increased fitness as a result of being able to respond to their environment.
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.
[9] 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.
The conservation of the development of neuronal tissue along the anterior-posterior axis provides insight into the evolutionary divergence of coelenterates and bilaterians.
Experiments have provided evidence that once neurons are differentiated, epithelial cell division drives their insertion into the nerve net.
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.
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.
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.
[13] Nerve nets contain intermediate neurons which allow for modulation of neural activity which occurs between the sensation of the stimulus and motor output.
