Nerve Net - Evolution

Evolution

Porifera is an extant phylum of the animal kingdom, as species in porifera do not have nervous systems. Porifera do not form synapses and myofibrils which allow for neuromuscular transmission, however, they do differentiate a proto-neuronal system and contain homologs of several genes found in cnidaria which are important in nerve formation. Sponge cells have the ability to communicate with each other via calcium signaling, or by other means; new research suggests that sponges express a group of proteins which conglomerate together to produce a partly synaptic formation. Larvae differentiate sensory cells which respond to stimuli including light, gravity, and water movement. In addition to sensory cells differentiated during development, adult porifera display contractile activity.

Nerve nets are found in species in the phyla cnidaria (e.g. scyphozoa, box jellyfish, and sea anemones), echinodermata, and ctenophora. Cnidaria and ctenophora, two animal phyla exhibiting radial symmetry, are collectively known as coelenterates. Coelenterates diverged 570 million years ago, prior to the Cambrian explosion, and they are the first two phyla that differentiated nervous systems based on synaptic conduction. Most research on the evolution of nervous tissue concerning nerve nets has been conducted using cnidaria as model organisms. The nervous systems of coelenterates allow for sensation, contraction, locomotion, and hunting/feeding behaviors. Coelenterates and bilaterians share common neurophysiological mechanisms, as such, coelenterates provide a model system for tracing the origins of neurogenesis. This is due to the first appearance of neurogenesis occurred in eumetazoa, which was a common ancestor of coelenterates and bilaterians. A second wave of neurogenesis occurred after the divergence of coelenterata in the common ancestor of bilateria. Although animals with nerve nets lack a true brain, they have the ability to display complex movements and behaviors.