Figure 2

Histology of the muscle system of N. vectensis . All sections are cross-sections. A Architecture of a single mesentery. The parietal muscle is located at the base of the mesentery, followed by a more distal retractor. Between the retractor muscle and the mesenterial filament at the tip of the mesentery, a gonad, embedded into the mesoglea, is formed. Inlet: Cross-section through the subpharyngeal region showing all eight mesenteries. B Detail of A. The parietal muscle shows a typical bilateral folding of the mesoglea, on which the myonemes of the muscle cells attach. The muscle consists of two contiguous sheets of myonemes, which merge into the ring muscle laterally. Two distinct neural plexi are visible on either side next to the parietal muscle. C Detail of A. In the retractor muscle the mesoglea folds and branches only to one side. This folding is generally more pronounced than in the parietal muscle. D Detail of A. The ring muscle lies between the parietal muscle of each mesentery. Similarly, the myonemes are located at the base of the cells. ep, epidermis; gd, gastrodermis; go, gonad; mef, mesenterial filament; mg, mesoglea; myo, myoneme; np, neural plexus; pam, parietal muscle; rem, retractor muscle; crm, column ring muscle; s, stalk. Scale: A, C: 100 μm; B, D: 50 μm; inlet: 1 mm. All sections are stained with Azan.