Which organism has the simplest nervous system?

Cnidarians, such as jellyfish and sea anemones, possess the simplest nervous system among the organisms listed. They have a decentralized nerve net rather than a centralized brain or complex structure. This nerve net allows them to respond to environmental stimuli, coordinate movements, and facilitate basic functions like feeding and locomotion.

The structure of a cnidarian's nervous system is suited to their lifestyle as simple, mostly sessile or free-floating organisms. The lack of a central nervous system supports their means of survival, such as reacting to predators or locating food, without the need for higher-level processing or complex behavior seen in more evolved organisms.

In contrast, the other groups listed have more complex nervous systems. Flatworms have a ladder-like nerve cord and some degree of cephalization, which allows for more coordinated movement and processing. Annelids and arthropods possess more developed nervous systems that involve a central nervous system with a brain, ganglia, and nerve cords, enabling them to perform more complex behaviors and functions.