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 with 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.