The animal kingdom is classified on the basis of
• levels of organization.
Segmentation: ...

Appendages: ...

Skeleton: ...

Sex: ...

Embryonic development: ...

Larvae:

The animal kingdom is classified based on several characteristics such as cell structure, body symmetry, presence or absence of a coelom, digestive system, reproduction, and evolutionary relationships. Let's discuss each of these characteristics in detail.

Cell structure:
This criterion is based on the presence or absence of a cell wall and the number of cells present in an organism. Animals can be classified into two types based on cell structure:

1. Unicellular animals: These are single-celled organisms that do not have a distinct nucleus and other organelles. Example: Amoeba.

2. Multicellular animals: These are organisms that have many cells and a complex organization of tissues and organs. Example: Humans.

Body symmetry:
This criterion is based on the symmetry of an organism's body plan. Animals can be classified into three types based on body symmetry:

1. Asymmetrical animals: These animals do not have any symmetry and have an irregular body shape. Example: Sponges.

2. Radially symmetrical animals: These animals have a circular or cylindrical body shape and can be divided into identical halves in any plane passing through the central axis. Example: Jellyfish.

3. Bilaterally symmetrical animals: These animals have a distinct left and right side and can be divided into two equal halves along the sagittal plane. Example: Humans.

Presence or absence of a coelom:
This criterion is based on the presence or absence of a fluid-filled cavity called a coelom. Animals can be classified into three types based on the presence or absence of a coelom:

1. Acoelomates: These animals do not have a coelom, and their organs are directly embedded in the mesoderm. Example: Flatworms.

2. Pseudocoelomates: These animals have a pseudocoelom, which is a body cavity that is not completely lined by mesoderm. Example: Roundworms.

3. Coelomates: These animals have a true coelom, which is a body cavity completely lined by mesoderm. Example: Humans.

Digestive system:
This criterion is based on the type of digestive system an organism has. Animals can be classified into two types based on their digestive system:

1. Incomplete digestive system: These animals have a single opening for both ingestion and egestion. Example: Hydra.

2. Complete digestive system: These animals have a separate mouth and anus for ingestion and egestion. Example: Humans.

Reproduction:
This criterion is based on the type of reproduction an organism has. Animals can be classified into two types based on their reproduction:

1. Asexual reproduction: These animals reproduce by themselves without the involvement of another individual. Example: Hydra.

2. Sexual reproduction: These animals reproduce by the fusion of gametes from two different individuals. Example: Humans.

Evolutionary relationships:
This criterion is based on the evolutionary relationships between different animal groups. Animals can be classified into several phyla based on their evolutionary relationships. Example: Chordata, Arthropoda, Mollusca, etc.

In conclusion, the animal kingdom is classified based on several characteristics such as cell structure, body symmetry, presence or absence of a coelom, digestive system, reproduction, and evolutionary relationships. These criteria help in understanding the diversity and relationships between different animal groups.