There is great diversity in the shape, structure, appearance in the body of different animals.However, the animals can be divided into three basic plans. These plans are shown below:
(1) The cell aggregate plan : It is found in the simple animals i.e., sponges. In these clusters of cells with a rudimentary division of labour is found in them.There are no tissues or organs. Nervous system is s also absent in them.
(2) The blind sac plan: It is found in coelenterates and flat worms. These animals have a digestive cavity with only one opening to the outside. Through this opening, food is ingested and undigested waste is are called symmetrical.
(3)The tube within a tube plan:It is found in more complex forms.In this plan body cavity forms one tube within which is situated another tube the alimentary canal;opening on one side by mouth and the other side of anus.