zoology - Is the appendix a vestigial structure in all vertebrates?

In humans the Appendix is a vestigial organ. Does it serve no apparent purpose in all the vertebrates that have one?

Answer

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 < 0.0001) phylogenetic signal in its distribution, and has been maintained in mammalian evolution for 80 million years or longer.