Foregut Formation of the Nemerteans and Its Role in Nemertean Systematics

Along a line of the four nemertean orders, Archi-, Palaeo-,Hetero-, and Hoplonemertea established by Iwata, formation ofthe foregut is described in detail on the basis of data hithertorecorded. The axiality of the egg, larva and adult worm of aspecies may change as a function of foregut morphogenesis inthe embryo. During foregut formation the mouth moves from thevegetal pole to the ventral side of the body. In Archinemertea,the larva has an angle of 60° formed by the long axis ofthe body and the foregut, while in Palaeonemertea the angleis 45°. This relationship is retained in the adult. Theplacement of the mouth thus obtained in the larva and adultworm seems to be important for the systematics of the Nemertea,especially for the Archinemertea and Palaeonemertea.