The diffuse nervous system is the most primitive nervous system. In diffuse systems nerve cells are distributed throughout the organism, usually beneath the outer epidermal layer. Large concentrations of nerve cells—as in the brain—are not found in these systems, though there may be ganglia, or small local concentrations of neurons. Diffuse systems are found in cnidarians (hydroids, jellyfish, sea anemones, corals) and in ctenophores, or comb jellies. However, the primitive nervous systems of these organisms do not preclude prolonged and coordinated responses and integrated behaviour to the simplest stimuli. An example is the movement of the sea anemone Calliactis onto the shell of the hermit crab Pagurus in response to a factor present in the outer layer of the empty mollusk shell occupied by the crab. This movement requires integration of the highest order.

Most cnidarians, such as those of the genus Hydra, have what is called a nerve net—a meshlike system of individual and separate nerve cells and fibres dispersed over the organism. Species of Hydra have two nets, one located between the epidermis and the musculature and the second associated with the gastrodermis. Connections occur at various points between the two nets, individual neurons making contact but not fusing, thereby forming structures similar to the chemically mediated synapses of vertebrates. Several specializations occur within various species. In Hydra the neurons are slightly more concentrated in a ring near the pedal disk and the hypostome (the “mouth”), but in jellyfish of a related genus the nerve fibres form a thick ring at the margin of the bell to form “through” conduction pathways.

The nervous systems of cnidarians correspond to their radially symmetrical bodies, in which similar parts are arranged symmetrically around a hollow gut cavity called the coelenteron. In some species nerve fibres course along the radial canals, where there may be arranged sensory bodies, called rhopalia, which contain ganglionic concentrations of neurons. In the sea anemone Metridium some of the nerve fibres are 7 to 8 mm (0.3 inch) long and form a system for fast conduction of nerve impulses. Such specializations may have allowed the evolution of different functions. Rapid coordination of swimming movements requires a fast-conducting pathway, while feeding relies on the nerve net. Integrative activity is likely to occur at the sensory ganglia, which may represent the first forms of a centralized nervous system.

The terminals forming synapselike structures in nerve nets contain synaptic vesicles that are thought to be packed with neurotransmitters and neuroactive peptides. Peptides present in Hydra nervous systems also exist in mammalian systems as neuromodulators, neurohormones, or even possible neurotransmitters.

Transmission in the nerve net is relatively slow compared with that in other nervous systems (0.04 metre per second in radial fibres of Calliactis compared with 100 metres per second in some fibres of the dog). Many repetitive stimuli may be required to elicit responses at these synapses. Long refractory periods are also characteristic of nerve nets, having durations about 150 to 300 times those seen at mammalian nerve fibres.

Finally, pacemaker systems are present in animals with nerve nets. In the sea anemone Metridium these systems are expressed in a series of spontaneous rhythmic movements that occur in the absence of any detectable stimulus. It is not known whether the movements originate from a “command” neuron or group of neurons or whether they arise without neuronal stimulation. It has been postulated that pacemaking cells were present in epithelial conducting systems known not to be nervous but that eventually evolved into neuronal tissue.

Centralized nervous systems

The development of the nerve net allowed an organism to engage in several different behaviours, including feeding and swimming. The development in the net of rapidly conducting bundles of fibres and of pacemaker systems allowed rapid withdrawal and rhythmic swimming activities, respectively, in some cnidarians. However, it is at the level of the flatworms (phylum Platyhelminthes) that there appears a longitudinal nerve cord and an anterior collection of nerve cells that can be called a brain. Furthermore, there are well-defined sensory and motor pathways as well as coordinating interneurons. Although nerve nets and pacemaker activity are still present in the flatworms, the presence of ganglia or a brain concentrated at the cephalic (head) end of the organisms represents a simple beginning to the complex centralized systems that develop at higher levels of the phylogenetic tree.

Simple bilateral systems

The flatworms were the first invertebrates to exhibit bilateral symmetry and also the first to develop a central nervous system with a brain. The nervous system of a free-living flatworm such as Planaria consists of a brain, longitudinal nerve cords, and peripheral nerve plexuses (interlacing networks of peripheral nerves; from Latin plectere, “to braid”). The brain, located in the anterior portion of the animal, is composed of two cephalic ganglia joined by a broad connection called a commissure. Longitudinal nerve cords, usually three to five pairs, extend posteriorly from the brain; they are connected by transverse commissures, and smaller, lateral nerves extend from the cords. The lateral nerves give rise to the peripheral nerve plexuses. The submuscular nerve plexus—consisting of sensory cells, ganglion cells, and their processes—is situated in the loose tissue (mesenchyme) below the subepidermal musculature. Another subepidermal plexus is located at the bases of the epithelial cells above the muscular layer.

Planaria are richly supplied with sensory receptors. Single sensory cells in the nerve plexuses are widely scattered over the organism. Sensory organs also are present and include ciliated pits and grooves, auricles, the frontal organ, statocyst, and eyes. The ciliated pits and grooves contain chemical receptors, or chemoreceptors, which permit the animal to detect food. The statocyst is responsible for balance and such reactions as rising to the surface of the water or sinking. The eyes, or ocelli, may occur as a pair situated anteriorly or may be scattered abundantly over the head region depending on the species. Short optic nerves connect the eyes with the brain.

Seven types of nerve cell bodies and two types of neuroglia have been described in Planaria. Removal of the brain results in the abolition of such functions as food finding and recognition and severe deficits in locomotion. However, the nerve cords by themselves can mediate a certain amount of locomotion as well as righting and avoidance reactions.

Nematodes (phylum Aschelminthes) have a high degree of centralization, with three-quarters of all nerve cells concentrated in a group of anteriorly placed ganglia and no peripheral plexuses or nets. They usually have eight longitudinal cords, commissures between dorsal and ventral cords, six cephalic nerves, a few special ganglia and nerves in the tail, and two sympathetic systems (one anterior and one posterior).