What type of nervous system is characterized by a nerve net?

A nervous system characterized by a nerve net consists of a decentralized and diffuse arrangement of interconnected nerve cells. This type of system is commonly found in simpler organisms, such as cnidarians (e.g., jellyfish and sea anemones), where there is no central brain or defined nerve cord structure. The nerve net allows for coordinated movements and responses to environmental stimuli without the need for centralized processing, enabling the organism to react to various stimuli from different directions simultaneously.

In contrast, a complex brain structure indicates a higher level of neurological organization not found in organisms with a nerve net. A system that allows for simple reflex actions suggests a more developed nervous system that typically involves a spinal cord or a centralized processing unit to mediate those reflexes, which is not applicable to the nerve net context. Similarly, a centralized nervous system, as seen in mammals, has a more sophisticated hierarchy and organization compared to the diffuse and interconnected nature of a nerve net. Therefore, the correct answer highlights the unique structure and function of a nerve net within simpler life forms.