Fundamentals of Animal Classification

Levels Of Body Organization

Following are the different levels of organization in animals from simpler to complex state:

1. Protoplasmic grade – In this grade all life activities are confined within the boundaries of a single cell which is a structural and functional unit of life. e.g. Protozoa and other unicellular organism.

2. Cellular grade – It is a loose association or aggregation of cells that are functionally differentiated, e.g. sponges.

3. Tissue grade – An aggregation of cells which act in co-ordination. e.g. Cnidarians.

4. Tissue-organ grade – When tissues aggregate to form a particular organ. First appeared in platyhelminthes.

5. Organ system grade – When organs work together to perform some common function such as digestion, respiration, circulation etc. e.g., most of the higher animals.