Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisation of cells.
For example, in sponges, the cells are arranged as loose cell aggregates, i.e., they exhibit cellular level of organisation. Some division of labour (activities) occur among the cells.
In coelenterates, the arrangement of cells is more complex. Here the cells performing the same function are arranged into tissues, hence is called tissue level of organisation.
A still higher level of organisation, i.e., organ level is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised for a particular function.
In animals like Annelids, Arthropods, Molluscs, Echinoderms and Chordates, organs have associated to form functional systems, each system 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.
Similarly, the circulatory system may be of two types:
(i) open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it or
(ii) closed type in which the blood is circulated through a series of vessels of varying diameters(arteries, veins and capillaries).