Figure 1

Overview of the naidid ground plan. A) Basic annelid body plan. The annelid body consists of an anterior non-segmental region composed of the prostomium (pr) and peristomium (pe), followed by a variable number of segments (grey bars), and a posterior non-segmental region, the pygidium (py). In front of the pygidium is the posterior growth zone (pgz), where new segments are made. B) Generalized structure of the nervous system in naidids. This schematic shows the anterior central nervous system (blue), ventral nerve cord neuropil (yellow) and peripheral nervous system (green). Anterior is to the left in this and all figures unless otherwise indicated. Labels: br: brain; cec: circumesophageal connective; con: interganglion connective; dch: dorsal chaetae; gut: ciliated gut; mo: mouth; pe: peristomium; pgz: posterior growth zone; phx: pharynx; pnI-IV: peripheral segmental nerve I-IV; pr: prostomium; prn: prostomial nerves; py: pygidium; sXg: segment x ganglion; seg: subesophageal ganglion; sep: intersegmental septum; vch: ventral chaetae.