Fig. 3

Notochords in phylogeny: enteropneusts, arthropods, phoronids, and cnidarians. a Enteropneust scenario (after Bateson [57]); the notochord homolog is the stomochord (sc); m mouth; a anus. b The inverted arthropod theory (after Patten [62]) with the notochord (no) ventral to the nerve cord (nc); m mouth; a anus. c, d The diplochord hypothesis (after Masterman [64]); c shows only the digestive system of the actinotroch larva (m mouth; a anus) with a pair of diplochords (dc). The cross section d (at the level of the dotted line in c) shows a diplochord (dc) on either side of the esophagus (eso). e–g Cnidarian hypothesis (after Lameere [69]). The sea anemone-like ancestor shown in side view (e) has an oblong mouth (m) leading to a pharynx (ph) and gastrovascular cavity (gvc); f shows the pharyngeal region mostly closing along the top and along the bottom to leave open, respectively, the neuropore (np) and the neurenteric canal (nec); in g, the notochord (no) has formed in the mesoglea between the nerve cord (nc) and the gastrovascular (gvc); arrows indicate the entry and exit of water. h–k Right-side up arthropod theory (after Gaskell [74]), starting with an arthropod (h) with appendages (ap), a primary gut (gut1), and a ventral nerve cord (nc); the nerve cord tissue migrates dorsally (i) and surrounds the primary gut (in j) to make a hollow, dorsal nerve cord (nc), while a secondary gut (gut2) forms by invagination from the ventral midline; subsequently (k), the secondary gut becomes the notochord (no) and a tertiary gut (gut3) is formed by the appendages fusing their tips. l, m Conversion of an arthropod-like enteropneust into a chordate (after Nübler-Jung and Arendt [87]). Before dorsoventral inversion (l), stomochord (sc) is not considered to be a notochord; the nervous system (in black) is annelid-like and includes a ventral nerve cord (vnc); in addition, a well-developed midventral mesentery, the pygochord (py) connects the gut to the ventral body wall; m mouth; a anus. After inversion (m, looped arrow), the anatomy becomes chordate-like as the original mouth disappears (asterisk), a new mouth (nm) opens and the old subesophageal ganglion migrates dorsally (arrows). In addition, the pygochord becomes the notochord (no) underlying the dorsal nerve cord (dnc)