The frenulum was thought to serve a structural role (buttress) in stabilizing the velarium during a swim contraction (Gladfelter, 1973), yet its position at the perradius
just opposite the rhopalial niche is well suited to receive direct motor commands from the adjacent rhopalium.
Each quadrant is radially subdivided at the perradius, midway between the interradii.
The structure of the velarium is not remarkable at the interradii, but it is supported by a buttress-like, subumbrellar frenulum at each perradius (Fig.
The raised band of smooth muscle at each perradius originates just below the nerve ring (Fig.
These bands run, at most, halfway to the perradius and terminate blindly in the subumbrellar tissue without fanning out (Fig.
The circular muscle sheet is continuous across each interradius, but is interrupted by a strip of radial, smooth muscle at each perradius. This perradial interruption separates the subumbrellar swim musculature into four quadrants that do not correspond to the four "flat sides" of the medusa.