The animals that have a true appendix include primates, the great apes, opossums and wombats. Other animals that have an appendix include mole-rats and rabbits. Many animals that do not have a true appendix still have an appendix-like structure called a cecum.

The appendix works the same way in animals as it does in humans. The appendix holds a large amount of lymphoid tissue. There are also masses of lymphoid tissue located in other gastrointestinal areas. These are known as gut-associated lymphoid tissues.

As of 2014 the function of the tissues is unknown, but researchers do know that they play a role in the ability of the body to recognize any foreign substances or immune triggers in food that is ingested. The lymphoid tissues found in the appendix have led to research that indicates that it possibly plays an important role in the body's immune system.

Although the cecum serves as a digestive organ in some animals, it still operates in the same manner that a true appendix does. The cecum features biofilms that are laid throughout the gut with a higher concentration of them near the very end of the gut. The biofilms hold the same beneficial bacteria that are found in the lymphoid tissue of the appendix.