The variety of annelid larval types and developmental modes reflects the high diversity and variability within these lophotrochozoans. However, our knowledge of pattern formation and tissue development in annelids and allies is scarce. In order to gain more data concerning neurogenesis and myogenesis in annelid trochophores, we analyzed different larval stages of Phyllodoce groenlandica using immunohistochemical staining techniques and subsequent confocal laser scanning microscopy (clsm). Focusing on pre-metamorphic stages, we reconstruct the process of tissue and body formation within planktonic polychaetous trochophore larvae. Our investigations revealed detailed knowledge of general developmental modes in Annelida and exhibit ontogenetic heterochrony of tissue development between two closely related annelid species. As such, P. groenlandica shows a delayed onset of nervous system development when compared with P. maculata. In contrast to the latter species, we were not able to detect the posterior sensory organ in larval P. groenlandica. We draw conclusions concerning general development of annelid trochophores and provide data showing new insights into the plasticity of body plans in Annelida. J. Exp. Zool. (Mol. Dev. Evol.) 320B:166–178, 2013. © 2013 Wiley Periodicals, Inc.