A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a condensation of neurons or, in more advanced forms, a central nervous system, organisms with radial symmetry are associated with nerve nets, and are found in members of the Ctenophora, Cnidaria, and Echinodermata phyla, all of which are found in marine environments. In the Xenacoelomorpha, a phylum of bilaterally symmetrical animals, members of the subphylum Xenoturbellida also possess a nerve net. Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.
It also exists in several other phyla, like chordates, annelids and flatworms, but then always alongside longitudinal nerve(s) and/or a brain.
The nerve net is the simplest form of a nervous system found in multicellular organisms. Unlike central nervous systems, where neurons are typically grouped together, neurons found in nerve nets are spread apart. This nervous system allows cnidarians to respond to physical contact. They can detect food and other chemicals in a rudimentary way. While the nerve net allows the organism to respond to its environment, it does not serve as a means by which the organism can detect the source of the stimulus. For this reason, simple animals with nerve nets, such as Hydra, will typically produce the same motor output in response to contact with a stimulus regardless of the point of contact.
The anatomy and positioning of nerve nets can vary from organism to organism. Hydra, which are cnidarians, have a nerve net throughout their body. On the other hand, sea stars, which are echinoderms, have a nerve net in each arm, connected by a central radial nerve ring at the center. This is better suited to controlling more complex movements than a diffuse nerve net.
Evolution
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. Recent taxonomic divisions, however, suggest that Ctenophora is sister to the other extant Metazoa.
Porifera is an extant phylum within the animal kingdom, and species belonging to this phylum do not have nervous systems. The placement of Ctenophora implies that either nervous systems were lost in the ancestor of Porifera, or they evolved independently in the ancestors of Ctenophora and ParaHoxozoa. 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. Sponge cells have the ability to communicate with each other via calcium signaling or by other means. Sponge larvae differentiate sensory cells which respond to stimuli including light, gravity, and water movement, all of which increase the fitness of the organism. In addition to sensory cells differentiated during development, adult Porifera display contractile activity.
The emergence of nervous systems has been linked to the evolution of voltage-gated sodium (Nav) channels. The Nav channels allow for communication between cells over long distances through the propagation of action potentials, whereas voltage-gated (Cav) calcium channels allow for unmodulated intercellular signaling. It has been hypothesized that Nav channels differentiated from Cav channels either at the emergence of nervous systems or before the emergence of multicellular organisms, although the origin of Nav channels in history remains unknown. Porifera either came about as a result of the divergence with Cnidaria and Ctenophora or they lost the function of the gene encoding Nav channels. As a result, Porifera contain Cav channels which allows for intercellular signaling, but they lack Nav channels which provide for the conductance of action potentials in nerve nets.