The primate appendix: a reassessment.

The presence of a vermiform appendix is often cited as a shared, derived character uniting the Hominoidea (apes and humans). However, appendix-like structures have been reported for many other primate taxa. A review of the literature reveals that the confusion arises because several different, and sometimes contradictory, criteria are enlisted to distinguish an appendix. The measures most frequently used to define this structure are gross shape and certain aspects of histology (e.g., lymphoid concentration). Unfortunately, descriptions of shape lack quantification, and histological thin-sections have not been studied for many primate taxa. In addition, although lymphoid concentration in the human appendix is known to vary considerably with age, this information is rarely reported in the primate literature. Given these complications, additional studies on the morphology and ontogeny of this region are warranted. This research will lead to a more accurate definition of the vermiform appendix. Most authors currently describe this feature as a narrow diverticulum of the cecum with thick walls and concentrated lymphoid tissue. However, the presence of thick mucosal layers and appreciable lymphoid tissue in taxa lacking appendices (e. g., Saguinus, Cercocebus) suggests that these features may be primitive primate traits. If so, wall thickness and lymphoid concentration cannot be used to define the vermiform appendix. These results suggest that a more rigorous definition of the appendix is requisite for this feature to be used in primate systematics.