Maslakova, S. A.*; Martindale, M. Q.; Norenburg, J. L.: Remains of the pilidium in direct developing nemerteans?

Nemertean worms display two basic types of development: direct and indirect. A specialized planktonic larva, the pilidium, characterizes the order Heteronemertea. The juvenile worm develops via a unique form of imaginal discs and sheds most of the larval body during radical metamorphosis. An encapsulated larva, homologous to the pilidium, is found in some heteronemertean species; in this case, metamorphosis takes place within the egg envelope. Species of two other major nemertean groups – the orders Palaeo- and Hoplonemertea have long been considered to have direct development, although a few literature sources point to the presence of larval epidermis, replaced by the definitive epidermis during an encapsulated development in several species. Whether a direct or an indirect type of development is ancestral is a key question in studies of evolution of the phylum. If this lost larval epidermis among palaeo- and hoplonemerteans can be shown to be homologous to that of the pilidium larva, and therefore plesiomorphic for the phylum, this would challenge the current, implicit view that direct development is primitive for Nemertea. Here we present the first photographs of the larval ectoderm of a hoplonemertean, as well as additional data on the distribution of the larval ectoderm among hoplonemertean species. Our recent cell-lineage studies provide some evidence that larval ectoderm of hoplonemerteans originates (at least in part) from the cells of the first quartet of micromeres. This supports the hypothesis that the larval ectoderm of hoplonemerteans is homologous to the larval body of the heteronemerteans, as the first quartet of micromeres produces most of the pilidial body.