The human vermiform appendix is probably not a "functionless sack" as many people assume. It presumably functions in providing an off-line inoculum of beneficial bacteria to the colon, when this activity of the caecum becomes compromised due to an extremely severe case of diarrhea or other gastro-intestinal disorder. Since the appendix's role in colon inoculation is also provided by the caecum, people can usually carry on typically normal lives without it, should it ever become inflamed and an appendectomy deemed necessary. While the functional significance of the appendix seems somewhat shrouded in redundancy, there is no reason for supporting that it is a primitive or degenerate structure. Although an "appendix-like" structure does exist in wombats, civets, rodents, and a few other lower animals, the appendix is evidently a specialized formation, unique to anthropoid apes and man, where it probably provides similar functions. Although the purpose described above is often omitted from textbooks, it does provide one possible explanation for its existence. It would not surprise me to hear that others do not necessarily agree with my interpretation of the appendix's functional significance, but I do know that I am not alone in my thoughts regarding it's possible role. Remember, this is science, and it is criticism and mystery that stimulate research and it is research that leads to new interpretations, new ways of thinking, and new questions to ask.

The appendix is a branch off of the cecum.The cecum is the pouch just below where the small intestine joins the large intestine. Originally, the cecum may have just added more area where things could be absorbed. In some animals, the cecum (ceca if they have more than one) is used as a place where plant material can be stored or fermented.
In humans the cecum and appendix don't seem to do anything. Most scientists feel that it is a leftover from earlier species. That is, maybe our ancestors ate more vegetation than we do and depended on a place where microbes could break down the food. Like all of the rest of the intestines, there are pockets of immune system tissue in the appendix. Some people have argued that the appendix isn't useless, but is important to the immune system. If you wanted to test whether the appendix were important to the immune (disease fighting) system, how would you do it?

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Good questions -- they frequently come up in discussions of comparative anatomy and evolution.

The human appendix (a small sac near the junction of the small and large intestine) is homologous to a structure called the "caecum", a large, blind chamber in which leaves and grasses are digested in many other mammals.

The appendix is often referred to as a "vestigial" structure. Presumably, as humans evolved, they no longer had need of a caecum yet it was not "lost" as part of the developmental program of humans. It would be interesting to find out if other primates have an appendix or a true caecum (or maybe nothing!). See if you can find this information and how it might influence the evolutionary aspect of how we (you) think about vestigial structures (organs useless to their present owners that serve important functions in other species).