MASLAKOVA, S. A. ; NORENBURG, J. L. : Trochophore larva is plesiomorphic for nemerteans: evidence for prototroch in a basal nemertean Carinoma tremaphoros (Phylum Nemertea, Palaeonemertea).

Nemertean worms are spiralians displaying a variety of developmental modes, from encapsulated direct to indirect planktotrophic development with radical metamorphosis. The pilidium, a specialized planktotrophic larva, characterizes the order Heteronemertea. Its juvenile develops via 5-7 ectodermal imaginal discs, and abandons the larval body in a drastic metamorphosis. Direct development characterizes the other two major groups of nemerteans – the orders Hoplo- and Palaeonemertea, and is either encapsulated or planktonic with a non-specialized planuliform lecitho- or planktotrophic larva. It was traditionally hypothesized that direct development is plesiomorphic for nemerteans. However, in several species of hoplo- and palaeonemerteans so-called larval ectoderm is lost or resorbed and ultimately substituted by definitive ectoderm. Earlier we suggested that this supports indirect development with drastic metamorphosis as plesiomorphic for the phylum. Here we present data on development of the palaeonemertean Carinoma tremaphoros. During early larval development large squamous cells, resembling those of larval ectoderm of other nemerteans, cover the larval surface except for the apical and posterior regions. Later, apical and posterior cells divide further, while these large cells are cleavage-arrested and form a skewed pre-oral belt. Based on morphology and position we hypothesize that this belt is a modified prototroch. We suggest that hoplonemertean larval ectoderm might be modified prototroch and that a trochophore larva bearing a prototroch is plesiomorphic for nemerteans.