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In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In (a) cnidarians, nerve cells form a decentralized nerve net. Nervous systems vary in structure and complexity. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species. These ganglia can control movements and behaviors without input from the brain.

It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). The insect nervous system is more complex but also fairly decentralized. 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. 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. Some organisms, like sea sponges, lack a true nervous system. Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure.