Define level of organition in animal kingdom

Although all members of Animalia are multicellular, they do not all display the same pattern of organization of cells. For example in sponges, cells are organized as loose cell aggregates (representing the cellular level of organization). There is some division of labor (activities) between cells. In covalents, the arrangement of cells is more complex.

Here cells performing similar functions are arranged in tissues, hence called tissue level of organization. An even higher level of organization, which is the organ level, is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form specific organs for a particular function. In animals such as annelids, arthropods, mollusks, echinoderms and chordates, organs are linked to form functional systems, each system related to a specific physiological function. This pattern is called the organ system level of the organisation.

Organ systems in different groups of animals exhibit different patterns of complexities. For example the digestive tract in Platyhelminthes has only one opening outside the body that serves as both the mouth and the anus, and is therefore said to be incomplete. A complete digestive system has two openings, the mouth and the anus. Similarly communication systems can be of two types:

1. The open type : in which blood is pumped from the heart and cells and tissues are directly bathed in it and

2. The closed type : in which blood circulates through a series of vessels of different diameters (arteries, veins and capillaries).