Animals usually are organisms. They are made up of complex organs forming systems that interact by way of chemical and electrical signals.
The most obvious is the make-up of any animal, the head, neck, body, legs, wings, or fins and tail. Each of these parts of an animal has specific functions and reasons they have evolved into their form. The largest organ of any animal is the skin.
The inside of an animal is made of up systems. These are the circulatory, muscular, nervous, skeletal, digestive, reproductive, endocrine and respiration systems.
The endocrine system is made up of glands. They are organs that excrete hormones and chemicals. It is triggered by signals from the nervous system.
No animal can function without these complex systems and the interaction within the body. They alert the animal of danger, hunger, the need for shelter and the need to mate. Animals that are weak in one system or organ will fall prey to a stronger animal.