Also found in: Dictionary, Thesaurus, Medical, Legal.
(also rudiment), an organ that has lost its basic significance in the process of the historical development of an organism. Rudimentary organs form during embryonic development but do not develop completely. Unlike larval organs, which are present only in embryos, rudimentary organs are retained for the entire lifetime of an organism. Rudimentary organs were normally developed organs in the ancestors of any given organism, although in future generations their functions are severely weakened or lost. In the latter case, the organs can apparently perform some other function.
Examples of rudimentary organs in animals include the fibula in birds, the eyes in some cave and fossorial animals, including Proteidae, mole rats, and moles, and the remains of the hairy covering and the pelvic bones in some whales. Rudimentary organs in man include the coccygeal vertebrae, the hairy covering of the torso, the ear muscles, the vermiform process of the cecum (appendix), and Morgagni’s ventricles.
Sometimes rudimentary organs develop completely, for example, the posterior pair of fins in whales. There have also been cases of humans being born with hair completely covering their bodies and faces. Rudimentary organs assist in establishing the course of phylogeny. Darwin believed that the existence of rudimentary organs was one of the proofs that man descended from animals.