In the region of the mouth where the two halves of the small arm-sinus approach one another they open into a central sinus lying beneath the oesophagus and partly walled in by the two halves of the ventral mesentery.
The intestine is slung by a median dorsal and ventral mesentery which divides the body cavity into two symmetrically shaped halves; it is " stayed " by two transverse septa, the anterior or gastroparietal band running from the stomach to the body wall and the posterior or ileoparietal band running from the intestine to the body wall.
This vessel is nothing but a split between the right and left folds of the mesentery, and its cavity is thus a remnant of the blastocoel.
A median mesentery running dorsoventrally supports the alimentary canal and is continued behind it into the tail, thus dividing the body cavity into two lateral halves.
Each mesentery is attached by its upper margin to the peristome, by its outer margin to the body-wall, and by its lower margin to the basal disk.