Nervous systems of invertebrate

 All cells have some stimuli that relay information both internally and externally. They have  no real nervous system, such as in the protozoa and sponges, but they have the coordination and reaction to external and internal stimuli. For example, the regular beating of protozoan cilia (see figure) or the response of flagellates to varying light intensities requires intracellular coordination. Only animals that) have true nervous systems. This clearly excludes the protozoa and sponges.

Some animals more complex than sponges, five general evolutionary level in nervous system id present. The first has been integrated throughout part two of this text. More complex animals possess more complex nervous systems.

Nervous system of cnidarian

The cnidarians (hydras, jellyfishes, and sea anemones)that  have the simplest form of nervous system. These animals have a nerve net, a framework that conducts impulses from one area to another (figure a). These nerve nets, have bidirectional impulse conduction by neuron. Cnidarians lack brains and but have local clusters of neurons. So, a nerve stimulus anywhere on the body initiates a nerve impulse that spreads across the nerve net to other body regions. In jellyfishes, this type of nervous system help to move slow swimming movements and in keeping the body right-side up.

Nervous system of Echinoderms

Echinoderms (e.g., sea stars, sea urchins, sea cucumbers) also have nerve nets, but they are complex. For example, sea stars have three distinct nerve nets. The First that lies just under the skin has a circumoral ring and five sets of nerve cords running out to the animal’s arms. Another net serves the muscles between the skin plates, that is the ossicles. The third net connects to the tube feet.

Nervous system of acoelomates

Animals, such as flatworms and roundworms, that have sense organs concentrated in the body region that first meet in new environmental stimuli. Thus, the second level in nervous system evolution involves cephalization, which is a concentration of receptors and nervous tissue in the animal’s anterior end. For example, a flatworm’s nervous system contains ganglia, which are distinct aggregations of nerve cells in the head region. Ganglia function as a primitive “brain” (figure b).

 Distinct lateral nerve cords (collections of neurons) on either side of the body carry sensory information from the periphery to the head ganglia and carry motor impulses from the head ganglia back to muscles, allowing the animal to react to environmental stimuli. These lateral nerve cords reveal that flatworms also exhibit the third level in nervous system evolution: bilateral symmetry.

 Bilateral symmetry have paired neurons, muscles, sensory structures, and brain centers. That help coordinated movements, such as climbing, crawling, flying, or walking.

In some invertebrates, such as crustaceans, segmented worms, and arthropods, the organization of the nervous system shows further advances. The axon in these invertebrates, join into nerve cords, and the size of it small, centralized brain, smaller peripheral ganglia help coordinate outer regions of the animal’s body.

 Ganglia can occur in each body segment or can be scattered throughout the body close to the organs they regulate (figure. Regardless of the arrangement, these ganglia represent the fourth evolutionary level. The more complex an animal, the more interneurons it has. Because interneurons in ganglia do much of the integrating that takes place in nervous systems, the more interneurons, the more complex behavior patterns an animal can perform. In echinoderms, such as starfishes, the nervous system is divided into several parts (figure f ).

 The ectoneural system retains a primitive epidermal position and combines sensory and motor functions. A radial nerve extends down the lower surface of each arm. A deeper hyponeural system has a motor function, and the apical system may have some sensory functions.

The fifth level in the evolution of invertebrate nervous systems is a consequence of the increasing number of interneurons. The brain consist of largest number of neurons, and have more complex the animal, and the more complicated its behavior, the more neurons are concentrated in an anterior brain and bilaterally organized ganglia. Vertebrate brains are an excellent example of this level.