The catch-up principle. Diagram of a cross-section of the velar edge of the gastropod Crepidula fornicata. The thick, compound cilia of prototroch and metatroch beat towards the band of single cilia of the adoral ciliary zone (food groove). The sequence of stages of the prototroch cilia are indicated by the small numbers. The prototroch cilia cut through the water, catch up with a food particle which is then pushed on to the adoral ciliary zone. This is apparently aided by the beat of the metatroch cilia. These cilia of the adoral zone carry the particles towards the mouth. From .