Similar body plans suggest common origins. Analogy - body parts with similar function , without regard to origin. Similar functions but different structures suggest different origins. These slits are found in the embryos of all vertebrates because they share as common ancestors the fish in which these structures first evolved. Human embryos also exhibit by the fourth week of development a well-defined tail, which reaches maximum length when the embryo is six weeks old.
Vertebrates retain traces of a feeding apparatus like that of tunicates and cephalochordates.
The gill slits, however, ceased to function as feeding structures, and then later as respiratory devices, as the vertebrate structure underwent evolutionary changes. The fishlike habitus that evidently began with cephalochordates became modified by the development of fins that were later transformed into limbs. Embryonic rudiments that never fully develop, such as the gill slits in humans, are common in all sorts of animals. Some, however, like the tail rudiment in humans, persist as adult vestiges reflecting evolutionary ancestry. The most familiar rudimentary organ in humans is the vermiform appendix.