Methorizis

in biology, the displacement of one rudimentary organ by another in the course of embryonic development. If the organ develops from several rudiments, the boundaries of the rudiments shift, and one rudiment more or less replaces the other. Thus, in the majority of crustaceans, most of the intestine develops from endoderm, whereas in isopods and decapods it develops from ectoderm. The term “methorizis” was proposed by the Russian zoologist V. M. Shimkevich in 1908.