Trends in Nervous System Evolution

What are the trends in nervous system evolution?

Nervous systems vary greatly in organization and complexity across the animal kingdom. Animal nervous systems exhibit different degrees of cephalization and specialization.

Invertebrates Invertebrate nervous systems range from simple collections of nerve cells to complex organizations that include many interneurons. You can see some examples in Figure 28–3.

▸ Nerve Nets, Nerve Cords, and Ganglia Cnidarians, such as jellyfishes, have simple nervous systems called nerve nets. As the name implies, nerve nets consist of neurons connected into a netlike arrangement with few specializations. In other radially symmetric invertebrates, echinoderms such as sea stars, for example, some interneurons are grouped together into nerves, or nerve cords, that form a ring around the animals' mouths and stretch out along their arms. In still other invertebrates, a number of interneurons are grouped together into small structures called ganglia (singular: ganglion), in which interneurons connect with one another.

▸ “Heads” As you learned in Chapter 25, bilaterally symmetric animals often exhibit cephalization, the concentration of sensory neurons and interneurons in a “head.” Certain flatworms and roundworms show some cephalization. Some cephalopod mollusks and many arthropods show higher degrees of cephalization. In these animals, interneurons form ganglia in several places. Typically, the largest ganglia are located in the head region and are called cerebral ganglia.

▸ Brains In some species, cerebral ganglia are further organized into a structure called a brain. The brains of some cephalopods, such as octopi, enable complex behavior, including several kinds of learning.

FIGURE 28–3 Invertebrate Nervous Systems Invertebrate nervous systems have different degrees of cephalization and specialization. Flatworms have centralized nervous systems with small ganglia in their heads. Cnidarians have a nerve net which, despite its simplicity, enables them to be successful predators (inset). Arthropods and cephalopod mollusks have a brain and specialized sensory organs.

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