an organ characteristic of the embryo or larva of a multicellular animal that disappears during the animal’s further development. Larval organs ensure the most important functions of an organism before it is formed and before the organs of the adult individual begin functioning. Examples are the abdominal limbs and gills of insect larvae; the gills, the horny mouthparts, and the tail of tadpoles; the vitelline vessels in the embryos of fish, reptiles, and birds; and the allantoic blood vessels in the embryos of reptiles, birds, and mammals.
An understanding of the structure and development of larval organs is helpful in determining the evolution of various groups of animals. Such knowledge is also used in the description of the ancestors of modern species; similar organs were characteristic of adult animals in a number of cases. Some larval organs of extant organisms, however, are embryonic adaptations to certain conditions of existence. Such adaptations, for example, the embryo sacs of Amniota, cannot be used to determine the structure of adult ancestors.