An organ is a collection of tissues that performs a particular function or set of functions in an animal or plant's body. The heart, brain, and skin are three organs found in most animals. The leaf, stem, and root are three organs found in most plants. Organs are composed of tissues, and may be organized into larger organ systems.
An accessory organ is an organ that assists with the functioning of some other organ within a system. Accessory organs include the teeth, tongue, salivary glands, gallbladder, liver, and pancreas. Organs, such as the vermiform appendix in humans, are termed vestigial organs.
An organ system is a collection of organs that have related roles in an organism's functioning. Eleven major organ systems are present within humans and animals, although some animals lack one or more of them. They are: