Is the appendix a vestigial structure in all vertebrates?
Smith et al. (2009) provide a survey of the morphology of the cecal appendix. One current hypothesis is that the appendix provides "safe harbor" for symbiotic gut bacteria. Among mammals, there is a vast array of cecal appendices:



In summary:


  A comparative anatomical approach reveals three apparent morphotypes
  of the cecal appendix, as well as appendix-like structures in some
  species that lack a true cecal appendix. Cladistic analyses indicate
  that the appendix has evolved independently at least twice (at least
  once in diprotodont marsupials and at least once in Euarchontoglires),
  shows a highly significant (P &lt; 0.0001) phylogenetic signal in its
  distribution, and has been maintained in mammalian evolution for 80
  million years or longer.