brains
Figure 35.2 Nervous systems vary in structure and complexity. In a cnidarians, nerve cells form a decentralized nerve net. In b echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as c planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, d arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and e octopi, which must hunt to survive, have complex brains containing millions of neurons. In f vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. credit e: modification of work by Michael Vecchione, Clyde F. Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH .