Fig. 4

A How somites might first have originated, as diverticula from the anterior medial wall of the metacoel (mt), in a fashion similar to the formation of the perihaemal diverticula in Fig. 3C, which are then replicated in series. The animal is modeled on the Cambrian fossil Gyaltsenglossus [38], a supposed hemichordate that is enteropneust-like in its vermiform body, but also bears a crown of tentacles which, assuming they are homologs of pterobranch tentacles, would be supported as shown by the mesocoel (ms). Contractions of the three nascent somites (in red), which would be paired, would flex the body side to side. B Swimming by flexing: a swimming sequence for the Spanish Dancer, Heterobranchus sanguineus, traced at 1 s intervals. Arrow indicates the point of maximal dorsoventral flexure, but this motion also allows the expanded margins of the mantle to take advantage of water currents. Currents account for most of the translational motion observed in this sequence, as the flexures themselves serve mainly to alter body posture.