Description of the Morphology of Pilidium incurvatum from Vietnam with Discussion of the Origin of Pilidia of the recurvatum Group (Nemertea, Pilidiophora)

Abstract

Pilidia from the recurvatum group are usually sock-like larvae and include the following types: pilidium recurvatum, pilidium incurvatum, and pilidium prorecurvatum. The origin of these pilidia remains debated.The external morphology, musculature, and the nervous system of pilidium incurvatum from Nha Trang Bay are described using light and confocal laser scanning microscopy. The morphologies of p. recurvatum and p. incurvatum are not significantly different. Unlike pilidium prorecurvatum, p. incurvatum has a telotroch, two ventral nerves, and a double ventral muscle strand; the nerve and the muscular rings are located close to the telotroch. The Dawydoff’s hypothesis about the origin of pilidia ex gr. recurvatum from a typical hat-like pilidium is supported.