Grades of organization:
We know that all organisms are made up of cells out of which some organisms may be unicellular while some may be multicellular. Grade of organisation refers to the levels in which the cells are arranged. There are different grades of organisation like:
Cellular level of organization: It is a loose aggregation of cells in which the cells are functionally different from one another.
Tissue level of organization: In this, different cells performing similar functions are arranged into tissues.
Organ level of organization: Here, different tissues are organized into organs and each organ is specialized for a particular function.
Members of the kingdom Animalia are multicellular and all of them exhibit the different pattern of organisation of cells. The different levels of organization are as follows:
Cellular level of organization (Example: Porifera)
Tissue level of organization (Example: Coelenterata)
Organ level of organization (Example: Platyhelminthes)
Organ system level of organization (Examples: Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata and chordates)
Symmetry is a characteristic through which animals may be distinguished from each other. Animals that can be divided into two identical halves in one plane exhibit bilateral symmetry. Animals that can be divided into many identical parts exhibit radial symmetry. Animals that cannot be divided into identical parts are asymmetric. If the body of an animal can be divided into two equal parts by any plane, it is said to be symmetrical.
(i) Animals showing radial symmetry − Hydra and sea anemone
(ii) Animals showing bilateral symmetry − Human beings and prawn