A:  The appendix is a thin pouch that projects out from the cecum, a part of the large intestine where the small intestine adjoins.  Other than the great apes, which are anatomically very similar to humans, few animals have cecal appendices.  Among them are rabbits, wombats, opossums, and mole-rats. 

Many animals species possess a much larger and well developed cecum than humans.  In some, the cecum serves as a place where fermentation of ingested food takes place.  It is thought in some circles that the small human appendix is more or less the result of evolutionary atrophy of the larger organ present in many animals, serving little purpose.  However, there are also some who argue that the appendix may have a unique role in maintenance of normal bacterial flora or in immune system health.