Fig. 1

The Nemertea and the diversity of larval forms. A The Nemertea comprises three major monophyletic groups, namely the Palaeonemertea, the Hoplonemertea and the Pilidiophora [99, 100]. The Palaeonemertea and Hoplonemertea show the so-called planuliform larvae, while the Pilidiophora shows as ancestral trait the presence of a planktotrophic pilidium larva (shown in B). This larva is present in the majority of lineages, although several groups exhibit several specializations, such as a shift towards lecithotrophic and/or encapsulated development. B Planktotrophic pilidium larva of Lineus longissimus. The typical pilidium has a hat-like shape, with an aboral apical organ and a ventral mouth surrounded by two long oral lobes. Internally, the larva has a blind gut, and as it matures, the imaginal discs that will form the adult grow inside. B Is a lateral view. Drawings are not to scale. The drawings of the planuliform larva of the Palaeonemertea and Hoplonemertea are adapted from [40]. ao apical organ, bg blind gut, mo mouth, ol oral lobes. Scale bar in B, 100 μm