The nervous system is a unique and essential feature of animals, originating from the primary germ layers during embryonic development. The notochord, which arises from the mesoderm, serves as a primitive backbone structure in chordates. In some species, this notochord develops into the vertebrae of the spine, while in others, it is a temporary structure that disappears as development progresses.

The neural tube, a hollow structure formed from the ectoderm, gives rise to the brain and spinal cord. As the ectoderm folds inward, it creates the neural tube, which swells in specific areas to form the embryonic brain. This process illustrates the evolutionary trend known as cephalization, where nervous tissue becomes concentrated at the anterior end of an organism, leading to the development of the brain, a mass of neurons responsible for integrating and processing sensory information.

In terms of organization, some animals possess a central nervous system (CNS), where nerves are clustered into one or more tracts that extend throughout the body. For example, a nerve cord can be seen in more complex organisms. Conversely, simpler animals often exhibit a nerve net, a diffuse arrangement of nerves found in radially symmetric organisms like starfish. This nerve net extends into the arms of the starfish, demonstrating a less centralized nervous system.

Segmentation is another important pattern observed in animal development, characterized by repeated body structures. For instance, worms display clear segmentation, and even organisms like flies exhibit segmentation during their developmental stages. The concept of segmentation is closely linked to homeobox genes, which play a crucial role in the developmental processes of various organisms.

In vertebrates, the vertebral column develops from the notochord and is segmented into vertebrae. Most vertebrates are classified as deuterosomes, while invertebrates, which lack a vertebral column, can still exhibit segmentation in their body structures. Many invertebrates are categorized as protostomes. A deeper exploration of vertebrates and invertebrates will be provided in subsequent chapters, offering more detailed insights into these two groups of organisms.