Figure 1

The muscle system of N. vectensis . Schematic overview of the muscle system in the adult polyp. The columnar musculature consists of the longitudinally orientated parietal and retractor muscle located in each of the eight mesenteries and a columnar ring muscle: All are of endodermal origin, the latter spanning the whole column between parietal muscles. In the tentacle an epidermal longitudinal muscle antagonises a gastrodermal ring muscle. Corresponding cross-sections of the polyp through the pharynx (1) and the subpharyngeal (2) region are shown to demonstrate the internal bilateral symmetry based on the arrangement of retractor muscles. crm, column ring muscle; gc, gastric cavity; m, mesentery; pam, parietal muscle; ph, pharynx; rem, retractor muscle; t, tentacle; tlm, tentacle longitudinal muscle; trm, tentacle ring muscle.