Video C. Ventral margin of the inner demibranch of Dreissena polymorpha as observed through the endoscope (looking dorsally). Polystyrene beads and the green alga, Secnedemus quadricauda, are moving anteriorly in the superficial mucous string. Particles approaching the ventral margin of the inner demibranch enter the mucous strings in different ways. Here, a particle (polystyrene bead, right) on the ascending lamella of the inner demibranch (upper) moves ventrally on the frontal surface of a filament. Once it reaches the filament tip, nearing the food groove, it moves laterally (to the left in this case) and enters the marginal groove proper through gaps between the filament tips.