Implantable devices, methods of implantation and tools for performing the implantations. A device includes a volume occupying member having a working configuration such that, when the device is implanted into the abdominal cavity of the patient, the device performs at least one of: prevention of expansion of the stomach of the patient into a space occupied by the volume occupying member in the abdominal cavity; and compression of a portion of the stomach. A device is configured to have substantially neutral buoyancy or a slightly positive buoyancy when implanted in the abdominal cavity external of the stomach. An attachment member is connected to the volume occupying member, and at least a portion of the attachment member is configured to encourage tissue ingrowth from at least one internal abdominal structure when held in contact therewith.