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Background  The pilidium larva is an idiosyncrasy defining one clade of marine invertebrates, the Pilidiophora (Nemertea, Spiralia). Uniquely, in pilidial development, the juvenile worm forms from a series of isolated rudiments called imaginal discs, then erupts through and devours the larval body during catastrophic metamorphosis. A typical pilidium is planktotrophic and looks like a hat with earflaps, but pilidial diversity is much broader and includes several types of non-feeding pilidia. One of the most intriguing recently discovered types is the lecithotrophic  pilidium nielseni  of an undescribed species,  Micrura  sp. “dark” (Lineidae, Heteronemertea, Pilidiophora). The egg-shaped  pilidium nielseni  bears two transverse circumferential ciliary bands evoking the prototroch and telotroch of the trochophore larva found in some other spiralian phyla (e.g. annelids), but undergoes catastrophic metamorphosis similar to that of other pilidia. While it is clear that the resemblance to the trochophore is convergent, it is not clear how  pilidium nielseni  acquired this striking morphological similarity.    Results  Here, using light and confocal microscopy, we describe the development of  pilidium nielseni  from fertilization to metamorphosis, and demonstrate that fundamental aspects of pilidial development are conserved. The juvenile forms via three pairs of imaginal discs and two unpaired rudiments inside a distinct larval epidermis, which is devoured by the juvenile during rapid metamorphosis.  Pilidium nielseni  even develops transient, reduced lobes and lappets in early stages, re-creating the hat-like appearance of a typical pilidium. Notably, its two transverse ciliary bands can be ontogenetically linked to the primary ciliary band spanning the larval lobes and lappets of the typical planktotrophic pilidium.    Conclusions  Our data shows that the development of  pilidium nielseni  differs remarkably from that of the trochophore, even though their larval morphology is superficially similar.  Pilidium nielseni ’s morphological and developmental features are best explained by transition from planktotrophy to lecithotrophy in the context of pilidial development, rather than by retention of or reversal to what is often assumed to be the spiralian ancestral larval type — the trochophore. Development of  pilidium nielseni  is a compelling example of convergent evolution of a trochophore-like body plan within Spiralia.    Electronic supplementary material  The online version of this article (doi:10.1186/s12983-017-0189-x) contains supplementary material, which is available to authorized users.

Development of a lecithotrophic pilidium larva illustrates convergent evolution of trochophore-like morphology