It is a basic structural and functions unit of life. Based on the no. of cell, organisms have been categorized as: Unicellular: Organisms whose body is made up of a single cell. E.g. amoeba, paramecium etc. Multicellular: Body is made up of multi (more than one) cells.
Protoplasmic Grade: 1. Animal having single cell 2. Also have cellular grade E.g. protozoans Cellular Grade: 1. Body having cellular level of organization 2. Cells are not co-ordinated to form tissue i.e. each cell perform a special function though the body has multi cells. E.g. Porifera
Many cells are co-ordinated to perform a designated/ specific function. E.g. nerve fibres of coelenterates
Different tissues combine to form organs. Different organs combine to form a system to perform a specific function. All the animals from phylum Platyhelminthes to phylum chordate exhibit organ-system grade of organization. In some systems are incomplete/absent.
Body coelom/Body cavity
Coelom is a cavity which lies between body wall (part of alimentary canal; intestine) Coelomates: Organisms having coelom in their body. Acoelomates: Organisms not having coelom in their body. E.g. protozoans, poriferans, coelenterates etc.