CBSE Class 9 Answered

Grade of organisation refers to the levels in which cells are arranged. There are different grades of oranisation like cellular (Porifera), tissue level (Coeleterata), organ level (Platyhelminthes) and organ system level (Nematoda) organisation. 

 

In imaginary sense, if body of any animal is cut through imaginary axis of body, it may or may not produce two equal halves. Depending upon this property, there are different types of animal bodies. This refers to symmetry. 

Asymmetrical Body: In case of such body, there is no any such imaginary axis of the body through which we can get two equal halves. Ex. Amoeba, Paramoecium, some sponges. Radial symmetry: In this type of body, if imaginary cut passes through central axis but any plane of body, it gives two equal halves. Ex. Star fish. Bilateral symmetry: In this type of body, there is only one such imaginary axis of body through which we can get two equal halves. Ex. Insects, fishes, frog, birds, human, etc.