An apparatus for treating obesity in a human or animal mammal patient. The apparatus comprising a first volume filling device segment and a second volume filling device segment. The first and second volume filling device segments are adapted to be assembled into an implantable volume filling device of a controlled size. Each one of the first and second volume filling device segment comprises at least one interconnecting structure. The interconnecting structure of the second volume filling device segment is adapted to be formed fitted, such that the first and second volume filling device segment can be assembled into the volume filling device. The assembled volume filling device is adapted to be at least substantially invaginated by a stomach wall portion of a patient, with the outer surface of the device resting against the stomach wall, such that the volume of the food cavity is reduced in size.