A narrow, elongated tube closed at one end, extending from the cecum, a blind pocket off the first part of the large intestine (see illus.). It is found in only a few mammals.The size and detailed structure of the appendix vary markedly, depending on the species and the age of the individual. Most reptiles, birds, and mammals have either a single or a paired cecum at the anterior end of the large intestine, but it is quite rare that this cecum has a thinner projection or true appendix.