Simple animals such as sea anemones can process information with simple networks of neurons that do little more than provide direct lines of communication from sensory cells to effectors (Figure 44.1a). The anemone's nerve net is most developed around the tentacles and the oral opening, where it facilitates detection of food or danger and causes tentacles to extend or retract. Bilaterally symmetrical animals, such as earthworms, that move more rapidly through their environments need to process and integrate larger amounts of information. This need is met by clusters of neurons called ganglia (Figure 44.1b). Ganglia serving different functions may be distributed around the body, as in the earthworm or the squid (Figure 44.1c). Frequently one pair of ganglia is larger and more central than the others and is therefore given the designation of brain.
In vertebrates, most of the cells of the nervous system are found in the brain and the spinal cord, the sites of most information processing, storage, and retrieval (Figure 44.1d). Therefore, the brain and spinal cord are called the central nervous system (CNS). Information is transmitted from sensory cells to the CNS and from the CNS to effectors via neurons that extend or reside outside of the brain and the spinal cord; these neurons and their supporting cells are called the peripheral nervous system.
Vertebrates differ greatly in their behavioral complexity and in their physiological specializations. Even the smaller and simpler nervous systems of invertebrates can be remarkably complex. Consider the nervous systems of small spiders that have programmed within them the thousands of precise movements necessary to construct a beautiful web without prior experience.
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