In animals like annelids, arthropods, mollusca, echinoderms and chordates, organs have associated to form functional systems, where each system is concerned with a specific physiological function. This pattern is called organ system level of organisation. Organ systems in different groups of animals exhibit various patterns of complexities.
For example, the digestive system in platyhelminthes has only a single opening to the outside of the body that serves as both mouth and anus and is hence called incomplete. A complete digestive system has two openings, mouth and anus.