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Bilateral symmetric animals are also seen in the nervous system from flatworms. Progress in the development of bilateral nerve systems can be summarized in the following evolutionary steps.
1. Increased centralization of the nervous system with the formation of the main nerve cords. These nerve cords (central nervous system, CNS) are regions in which the cell body of many neurons in, or near, the majority of paths between receptors and effectors passes.
2. Limitation of transmission in nerve paths in only one direction. In this way, a natural distinction has been made between sensory wires (affernet wires) leading to CNS and motor wires (eferent wires) exiting the CNS.

3. Increase in the complexity of nerve pathways in CNS, with the addition of more dialysons. This development has increased the flexibility of the response.
4. Increasing separation of cells with different functions within the nervous system. Thus, separate functional areas and structures became evident.
5. The formation of the brain as a result of the progressive upward development of the anterior end of the cord and the progression of cephalization (head formation).
6. Increased number of complex organs and complexity. These steps are not evident in many primitive flatteners (those considered to be ancestral forms). These flatwaters only have a neural network similar to the hydradakine. Some of the more advanced neural systems are formed by the grouping of neurons in the neural network and the large nerve cords (strips) extending from the front to back are seen. These cords can be eight and are located ventrally, dorsally and laterally. In more advanced flatteners, this lengthwise decrease is observed. In their most advanced, there are two of them, and they are located ventrally.
Longitudinal cords, the least developed flatteners at the front end show very little specific structure signs; but in spite of this, small bulges can be regarded as a lik brain ├╝yle with some tolerance. At the more advanced end of the spectrum, flatteners have a much better developed brain, but this brain has only a limited dominance over other parts of the central nervous system .
It is no coincidence that an animal's brain is almost always at the front end of its body. The evolutionary and functional explanation of this naturally visible settlement of the brain must be related to the animal's direction of movement. The anterior portion is usually the first part of a bilateral animal experiencing a stimulus during movement. Thus, natural selection has affected the development of the sensory organ at a very high density in this region; this led to the extension of the anterior end of the longitudinal nerve cord.
Probably the most primitive form of the brain was related to collecting the stimuli from the sensory organs on the nerve strings, carrying the information to the appropriate motor neurons. The selective advantage of comparing the various sensory stimuli, that is, processing the incoming information before transmitting to the muscles, should have led to an increase in the number of intermediate neurons in the anterior region where sensory receptors have already been concentrated. Thus, as can be seen in an organism, such as a nematode, the brain has become an area of ​​analysis rather than a simple collection of sensory cells transmitting raw sense information to further effectors. With its increased specificity in the analysis, the brain has also become a coordination center and its dominance on other parts of the CNS has also increased.
Evolutionary development, which is evident in yassisolucan, has reached the highest level of development in vertebrates (especially in mammals) and in high invertebrates such as molluscs, molluscs, cephalopods and arthropods . All of these animals have a high level of centralization, and the old neural network strategy is represented by structures that only face a small number of blindness. These structures are found in the regions where the body is controlled by slow and diffuse conduction of slow movements similar to intestinal peristaltic contractions in mammals.
In high invertebrates, the central nervous system is a double elongated cord which is located ventrally. Within them, the bodies of neurons form ganglia and the wires gather in the nerves and form the communication pathways between the ganglia. Even in primitive rings, there are obvious ganglion masses. They are located as a pair in each body segment and are interconnected by nerves extending between segments.
Almost all cell bodies were located in these ganglia. The brain is another ganglion at the head of the animal. The invertebrate's brain is normally larger than the ganglions in the segments as it occurs by combining the most prominent four ganglia, and the sensory neurons in the mixed cells occupy more space than the motor neurons. The dominance over other ganglia is remarkable, but this dominance is limited compared to the vertebrate brain.
More advanced arthropods, especially some insects, show more intense coordination at the front end. Furthermore, most of the other segmental ganglia are combined, resulting in better control integrity between segments. On the other hand, the brain is proportionally small and the chest ganglia (combined with some abdominal ganglia in many organisms) act as many vital coordinates. The presence of chest ganglia in insects is probably related to at least two major body arrangements, legs andthe wings are attached to the thorax and therefore are conducive to the condensation of the coordinating motor centers in the thorax. The second is that most of the sense organs are located on the legs and thorax. (for example, the flies are on the legs of the taste receptors - so that they know better what they are walking on, and in many insects the hearing organs are found in the thorax or legs).
Several insects are still found to function autonomously in various ganglia. For example, the neural ik information “required for behavioral programs that control walking, flying, flirting, mating and insertion is stored in the thoracic and abdominal ganglia, and the task of the brain is to direct, initiate, or terminate behavior. They can even learn flies and cockroaches without head.
Source: poxox blogs

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