Mention the characteristics taken as basis for the classification of animals.

Explanation

### Step-by-Step Text Solution:

1. **Structural Organization**: The first characteristic considered in the classification of animals is their structural organization. This refers to the complexity of the body structure, ranging from simple unicellular organisms to complex multicellular organisms.

2. **Body Symmetry**: The second characteristic is body symmetry. Animals can be classified based on their body symmetry into three main types: bilateral symmetry (where the body can be divided into two identical halves), radial symmetry (where the body can be divided into multiple identical sections around a central axis), and asymmetry (where there is no symmetry).

3. **Presence of Notochord**: The third characteristic is the presence or absence of a notochord. A notochord is a flexible rod-like structure found in some animals during their development. Animals that possess a notochord are classified as chordates.

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