Final answer:
Animal nervous systems vary from the simple nerve net in jellyfish to the highly complex structures in vertebrates. Flatworms and insects possess basic CNS and peripheral ganglia, while octopi have advanced nervous systems with specialized lobes and vertebrate-like eyes. Vertebrates have a centralized CNS with a brain and a spinal cord, and show higher levels of cephalization.
Step-by-step explanation:
The nervous systems of animals exhibit remarkable diversity and complexity in structure. For example, jellyfish have a simple nerve net instead of a centralized brain, and flatworms feature a rudimentary central nervous system (CNS) with a cerebral ganglia acting as a simple brain with two nerve cords. Insects have a more elaborate system including a ventral nerve cord and multiple ganglia capable of controlling behaviors autonomously. In contrast, octopi possess a highly complex CNS with specialized lobes and advanced eyes, resembling vertebrate sensory structures. Vertebrates, with a more centralized and specialized nervous system, have a CNS consisting of a brain and dorsal spinal cord, and a peripheral nervous system (PNS) comprising sensory and motor nerves.
Cephalization is the evolutionary trend of concentrating sensory organs and nerve cells at the anterior end of the body, resulting in a head region with centralized brain structures, which is prominent in vertebrates and some invertebrates.