Classification of animals is based upon all of the following except :

Options

A. level of organisation
B. type of body plan
C. type of coelom
D. presence or absence of a nervous system

Explanation

To solve the question regarding the classification of animals, we will analyze each option provided and determine which one is not a criterion for classification.

### Step-by-Step Solution:

1. **Understand the Question**: We need to identify which of the given options is NOT a basis for the classification of animals.

2. **Evaluate the First Option: Level of Organization**:
- Animals can be classified based on their level of organization.
- They are categorized into diploblastic (two germ layers: ectoderm and endoderm) and triploblastic (three germ layers: ectoderm, endoderm, and mesoderm).
- **Conclusion**: This option is a valid criterion for classification.

3. **Evaluate the Second Option: Type of Body Plan**:
- Animals are classified based on their body plan, which includes symmetry.
- The types of symmetry include radial symmetry, bilateral symmetry, and asymmetry.
- **Conclusion**: This option is also a valid criterion for classification.

4. **Evaluate the Third Option: Type of Coelom**:
- Coelom refers to the body cavity between the body wall and the digestive tract.
- Animals can be classified as:
- Acoelomates (no coelom)
- Pseudocoelomates (false coelom)
- Coelomates (true coelom)
- **Conclusion**: This option is a valid criterion for classification.

5. **Evaluate the Fourth Option: Presence or Absence of a Nervous System**:
- While the presence or absence of a nervous system is an important characteristic of animals, it is not a primary basis for their classification.
- Animals are not systematically classified solely based on whether they have a nervous system or not.
- **Conclusion**: This option is NOT a valid criterion for classification.

6. **Final Answer**: The classification of animals is based upon all of the following except the presence or absence of a nervous system.