Here you go Wayland, this explains it:
The vermiform appendix: background info
In humans, the vermiform appendix is a small, finger-sized structure, found at the end of our small caecum and located near the beginning of the large intestine. The adjective "vermiform" literally means "worm-like" and reflects the narrow, elongated shape of this intestinal appendage. The appendix is typically between two and eight inches long, but its length can vary from less than an inch (when present) to over a foot. The appendix is longest in childhood and gradually shrinks throughout adult life. The wall of the appendix is composed of all layers typical of the intestine, but it is thickened and contains a concentration of lymphoid tissue. Similar to the tonsils, the lymphatic tissue in the appendix is typically in a constant state of chronic inflammation, and it is generally difficult to tell the difference between pathological disease and the "normal" condition. The internal diameter of the appendix, when open, has been compared to the size of a matchstick. The small opening to the appendix eventually closes in most people by middle age. A vermiform appendix is not unique to humans. It is found in all the hominoid apes, including humans, chimpanzees, gorillas, orangutans, and gibbons, and it exists to varying degrees in several species of New World and Old World monkeys.
The caecum: a specialized herbivorous organ
Our appendix is a developmental derivative and evolutionary vestige of the end of the much larger herbivorous caecum found in our primate ancestors. The word "caecum" actually means "blind" in Latin, reflecting the fact that the bottom of the caecum is a blind pouch (a dead-end or cul-de-sac).
In most vertebrates, the caecum is a large, complex gastrointestinal organ, enriched in mucosal lymphatic tissue and specialized for digestion of plants. The caecum varies in size among species, but in general the size of the caecum is proportional to the amount of plant matter in a given organism's diet. It is largest in obligate herbivores, animals whose diets consist entirely of plant matter. In many herbivorous mammals the caecum is as large as the rest of the intestines, and it may even be coiled and longer than the length of the entire organism (as in the koala). In herbivorous mammals, the caecum is essential for digestion of cellulose, a common plant molecule. The caecum houses specialized, symbiotic bacteria that secrete cellulase, an enzyme that digests cellulose. Otherwise cellulose is impossible for mammals to digest.
The structure of the caecum is specialized to increase the efficiency of cellulose fermentation. As a "side branch" from the gut it is able to house a large, dense, and permanent colony of specialized bacteria. Being a dead-end sac at the beginning of the large intestine, it allows more time for digesting food to reside in the gut and ferment more completely, before passing through the large intestine where the resulting nutrients are absorbed. However, even though humans are herbivorous, the small human caecum does not house significant quantities of cellulase-excreting bacteria, and we cannot digest more than but a few grams of cellulose per day (Slavin, Brower, and Marlett 1980).
The pink highlighted bits are appendixes
(Borrowed with thanks from: http:/www.talkorigins.org/faqs/vestiges/appendix.html )