What system carries signals throughout the body to communicate messages?

The nervous system is the primary system responsible for carrying signals throughout the body to communicate messages. It consists of the brain, spinal cord, and a network of nerves that connect different parts of the body. When a stimulus occurs, sensory receptors send electrical signals to the brain, which processes the information and then sends out appropriate responses through motor neurons. This rapid communication allows the body to react to changes in the environment, coordinate actions, and manage bodily functions.

In contrast, the excretory system is involved in waste removal, the muscular system facilitates movement, and the circulatory system transports blood, nutrients, and oxygen throughout the body but does not primarily serve to transmit nerve signals. Hence, the nervous system is specifically designed for communication and signal transmission, making it the correct choice for this question.