true brain
Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 35.2. Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells neurons called a nerve net. Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves. Flatworms of the phylum Platyhelminthes have both a central nervous system CNS , made up of a small brain and two nerve cords, and a peripheral nervous system PNS containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia clusters of connected neurons . These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systemsthey have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.