In some forms at least, pores between (not through) the ambulacral elements, or between them and the thecal plates, seem to have permitted the passage of extensions from the perradial water-vessels.
Ra, Tentaculocysts, four perradial, four interradial.
- Surface view of the Subumbrella or oral aspect of Aurelia aurita, to show the position of the openings of the subgenital pits, GP. In the centre is the mouth, with four perradial arms corresponding to its angles (compare fig.
The transparent prolonged into the four long tissues allow the enteric cavities and oral arms, perradial in position.