How many classes are there in the subphylum vertebrate? list 3 classes

It is the nervous system! The nervous system is like the control center for the rest of the body. So normally, it sends signals to the rest of the body so it can function properly due to external or internal circumstances. Lets say you are exercising. You start to sweat. Why? The brain is sending signals to the appropriate part of the body to let the function of sweat begin due to your internal and external changes.