The mollusk's feeding system starts with a rasping "tongue", the radula. The complex digestive system uses mucus and microscopic, muscle-powered "hairs" called cilia. The generalized mollusk has two paired nerve cords, or three in bivalves. The brain, in species that have one, circles the oesophagus. Most mollusks have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults.