Nerve net : A topographic division that is a longitudinally arranged nervous system (Monro, 1783) with radial symmetry and relatively evenly and diffusely distributed neurons (Waldeyer, 1891), characteristic of Coelenterates, Ctenophores, Hemichordates, Pogonophores, and adult Echinoderms. While small neuron condensations (called marginal ganglia) and circular zones (called nerve rings) with a higher concentration of neurons and neuron extensions can be present, nerves (Herophilus, c335-280 BC) are not clearly differentiated, and nothing like a central nervous system (Meckel, 1817) is present; see Bullock & Horridge (1965, p. 12 & Ch. 8), Cobb (1995). It is thus a nervous system with no clear topographic divisions-a divisionless nervous system. The nervous system of hydra (a Coelenterate) is an example of a relatively simple diffuse nerve net, whereas the terminal plexuses in the wall of the mammalian digestive system can be viewed as analogous to a relatively differentiated nerve net.