CBSE Class 11 Biology Chapter 6: Animal Kingdom NCERT Solutions

If common fundamental features are not considered during the classification of animals, significant difficulties would arise. The primary objective of classification is to group organisms based on shared characteristics to understand their relationships and diversity. If only specific, non-fundamental features were used, each organism might end up in its own distinct group. This would make the entire classification system inefficient and meaningless, as it would fail to reveal evolutionary connections or allow for comparative studies between different groups of animals. Consequently, understanding the evolutionary significance and comparative anatomy of different species would become extremely challenging.

To classify an animal specimen, a systematic approach using fundamental features is followed:

1. Observe the Level of Organization: Determine if the organism exhibits a cellular level (like sponges), tissue level (like cnidarians), organ level (like platyhelminthes), or organ system level (most other animals).
2. Identify Body Symmetry: Check for radial symmetry (body parts arranged around a central axis, e.g., cnidarians) or bilateral symmetry (body can be divided into two equal left and right halves, e.g., annelids, arthropods).
3. Examine the Body Cavity (Coelom): Ascertain the presence and nature of the body cavity. Animals can be:
   • Acoelomate: Lacking a body cavity (e.g., Platyhelminthes).
   • Pseudocoelomate: Having a body cavity not completely lined by mesoderm (e.g., Aschelminthes).
   • Coelomate: Possessing a true coelom, a body cavity completely lined by mesoderm (e.g., Annelida, Arthropoda, Chordata).
4. Note Other Features: Depending on the specimen, other characteristics like segmentation, presence of a notochord, or specific organ systems might be observed.

By systematically evaluating these features, the specimen can be placed into its appropriate taxonomic category.

The study of the body cavity and coelom is highly useful and serves as a critical criterion in the classification of animals. The coelom is defined as a fluid-filled space or cavity located between the digestive tract and the outer body wall. Its presence, absence, and the way it is lined significantly help in distinguishing between major animal groups (phyla).

• Coelomates: Animals possessing a true coelom, which is entirely lined by mesoderm, belong to this category. Examples include Annelids, Mollusks, Arthropods, Echinoderms, and Chordates. The coelom plays vital roles in circulation, excretion, and providing hydrostatic support.
• Pseudocoelomates: In these animals, the body cavity is present but is not completely lined by mesoderm. The mesodermal cells are scattered in the space between the digestive tract and the body wall. Aschelminthes (roundworms) are a prime example.
• Acoelomates: Animals that lack a body cavity altogether are known as acoelomates. The space between the digestive tract and the body wall is filled with solid cells. Platyhelminthes (flatworms) are examples of acoelomates.

Therefore, the nature of the body cavity (coelom, pseudocoelom, or absence thereof) is a fundamental characteristic used to establish evolutionary relationships and classify animals into distinct phyla.