The feeding current itself is generated by the lateral cilia. There are no cilia on the outside edges of the tentacles. Relax the animal in isotonic magnesium chloride and remove it from its tube. The tentacles are held errect by hydrostatic pressure in the mesocoel which is generated by muscles in the body wall. Muscles cannot contract in the presence of magnesium because this ion interferes with the calcium dependent acto-myosin system of muscles, presumably by replacing calcium ions.
Magnesium has no effect on the tubulin-dynein system of cilia which is independent of calcium.
Use the lophophore to orient yourself. It is located on the ventral side of the anterior end of the animal. Spread its tentacles apart and look for the mouth on the midline of the central, unscrolled region of the lophophore. The mouth is crescent-shaped and, of course, is inside the lophophore. The mouth is displaced to the ventral side of the anterior end.