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Foregut Formation of the Nemerteans and Its Role in Nemertean Systematics

Fumio Iwata
American Zoologist
Vol. 25, No. 1 (1985), pp. 23-36
Published by: Oxford University Press
Stable URL: http://www.jstor.org/stable/3882807
Page Count: 14
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Foregut Formation of the Nemerteans and Its Role in Nemertean Systematics
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Abstract

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

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