Discuss various level of organisation in animal kingdom. 

Explanation

All members of Animalia are multicellular heterotrophic eukaryotes.
But all of them do not exhibit the same pattern of organisation of cells.
The cells in their body are of several types.
These are organised into several functional units of progressively increasing complexity.
The animal body shows four basis levels of structural organisation i.e. given below:
  (1) Cellular Level:
In this level, the body shows some division of labour among cells.
It is found in sponges.
The body consists of many cells arranged as loose cell aggregates but the cells do not form tissue.
Tissue Level Organization :
In coelenterates the arrangement of cells is more complex.
The cells performing the same function are arranged into tissues, hence is called tissue level of organisation.
Organ Level:
In Platyhelminthes and other higher phyla tissues are grouped together to form organs, each specialised for a particular function. i.e. organ level organisation is present.
(4) Organ System Level:
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
(ii) Closed type : In which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).