Fig. 14

Internal versus external VOs and reconstruction of the plesiomorphic VO type in crown group Pycnogonida. a Schematic representation of external and internal VOs from postlarval to juvenile instars, showing an idealized sagittal section. Lightly-colored, larger cells in the VOs represent the NSC-like precursors. Black cells detach from the VOs and start migration. Yellow cells are neural precursors that divide again while they migrate into the soma cortex (as shown in [22]). b Mapping of the VO types on two competing hypotheses on the phylogenetic relationships of extant pycnogonid taxa. Although several lineages remain unstudied (labeled with question marks), the distribution of the two VO types across the taxa investigated suggests internal VOs to be the plesiomorphic condition of Pycnogonida. Abbreviations: GN – glomerulus-like neuropil, VLT – ventral longitudinal tract, VO – ventral organ