Fig. 1

Evolution of larval development in nemerteans. The pilidium larva, defined here as a complex character including development via imaginal discs and juvenile rudiments, a larval body distinct from the juvenile body, an inflated blastocoel, catastrophic metamorphosis and distinct ciliary bands (black), is only found in the Pilidiophora. Hubrechtids possess a helmet-like planktotrophic pilidium with all of the listed features. Heteronemerteans display a great diversity of pilidia, including lecithotrophic forms, such as pilidium nielseni. Lecithotrophic pilidia lack inflated blastocoel; a few also lack distinct ciliary bands. Nevertheless, all pilidiophoran larvae possess other essential pilidial traits. Hoplonemerteans and palaeonemerteans possess uniformly ciliated juvenile-like larvae with small blastocoel and gradual metamorphosis. Hoplonemertean larvae possess a transitory larval epidermis (hence the name “decidula”), and lack any prototroch vestiges. At least one palaeonemertean genus, Carinoma, possesses a vestigial prototroch (grey), derived from the spiralian trochoblast lineage (including 1q² cells). Parsimony suggests that pilidium evolved at the base of the Pilidiophora, while juvenile-like uniformly ciliated larvae are ancestral to Nemertea. Thus the distinct ciliary bands of pilidiophoran larvae (including those of pilidium nielseni) must have evolved independently from the ciliary bands (including the prototroch), of other spiralian larvae, even if the prototroch, and the trochophore larva defined by it, are shown to be ancestral to the Trochozoa, or Spiralia, as a whole. It is not possible to represent the immense diversity of spiralian larvae in this figure. The goal here is merely to highlight that a trochophore larva defined by the presence of a preoral differentially ciliated prototroch derived from the trochoblast lineage is found in at least some annelids and mollusks. Note that the eye spots, as depicted here, do not reflect morphology of any particular species, but simply mark the position of the juvenile head