sentences: the digestive system is a part of an animal
docs: bird_digestive_tract is body_part. mammal_digestive_tract is digestive_tract. mammal_digestive_tract is body_part. digestive_system is system. reptile_digestive_tract is body_part. organ_system is organism_part. digestive_tract is organ_system. digestive_tract is body_part. all_animals has digestive_system mammal_rib_cage is body_part.