(iii) Cell-tissue (iv) Tissue-organ (v) Organ-system.
1. Protoplasmic or acellular level: This type of organization is found in protozoa or acellular protista. All the activities are performed by single celled body. However, specialized cytoplasmic structures, or the organelles carry on specific functions, thus illustrating division of labour.
2. Cellular level: Multicellular animals such as mesozoans and sponges, are made of loose association of cells. All the cells are in direct contact with the environment, i.e., water which facilitate respiration, excretion and other activities to etc.
take place by the general body surface. Cellular specialization however occurs but cells do not show coordination with each other. 3. Cells-tissue level: Coelentrates are supposed to make the beginning of the tissue plan of the body. They posses interconnected nerve net of nerve cells with the function of transmission of impulses and coordination.
Thus first truly metazoan phylum of tissue grade organization is Cnidaria or Coelenterata. Their body is generally radially symmetrical and made of only two germ layers, ectoderm and endoderm. Such animals are called diploblastic. 4. Tissue-organ level: Platyhelminths are the first bilaterally symmetrical, acoelomate metazoans to have reached the tissue-organ level of organization. They have a number of well-defined organs such as eyespots, proboscis, gonads etc. They are also triploblastic because they have a third or middle cellular germ layer called mesoderm, which lies between outer ectoderm and inner endoderm. 5.
Organ system level: In higher animals several organs are associated to form a distinct system concerned with a specific function like digestion, respiration, circulation, excretion and reproduction. This is the highest level of organization. This is first seen in a group of marine worms known as Nemerteans, Aschelminthes, Annelids and all the other higher phyla show this kind of organization.