Animal Kingdom Chapter 4 Class 11 Biology NCERT Textbook PDF

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Animal Kingdom

Chapter 4: Animal Kingdom

4.1 Basis of Classification

In spite of the differences in structure and form of different animals, there are fundamental features common to various individuals in relation to the arrangement of cells, body symmetry, nature of coelom, and patterns of digestive, circulatory, or reproductive systems. These features are used as the basis of animal classification and some of them are discussed here.

4.1.1 Levels of Organisation

Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisation of cells.

For example, in sponges, the cells are arranged as loose cell aggregates, i.e., they exhibit the cellular level of organisation.

Some division of labor (activities) occurs among the cells. In coelenterates, the arrangement of cells is more complex.

Here the cells performing the same function are arranged into tissues, hence is called tissue level of organization.

A still higher level of organization, i.e., organ level is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialized
for a particular function.

In animals like Annelids, Arthropods Molluscs. Echinoderms and Chordates, organs have associated to form functional systems, each the system concerned with a specific physiological function.

This pattern is called an organ system level of organization. Organ systems in different groups of animals exhibit various patterns of complexities.

For example, the digestive system in Platyhelminthes has only a single opening to the outside of the body that serves as both mouth and anus and is hence called incomplete.

4.1.2 Symmetry

Animals can be categorized on the basis of their symmetry. Sponges are mostly asymmetrical,
i.e., any plane that passes through the center does not divide them into equal halves.

When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry.

Coelenterates, ctenophores, and echinoderms have this kind of body plan (Figure 4.1a).

Animals like annelids, arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.

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NCERT Solutions Class 11 Biology Chapter 4 Animal Kingdom

1. What are the difficulties that you would face in the classification of animals if common fundamental features are not taken into account?


Animals are classified on the basis of common features such as cell arrangement, body symmetry, nature of coelom, digestive tract, circulation, or reproductive system.

Without these common features, it is very difficult to treat each organism individually. It is impossible to add new species detected every day without common features. To study diversity in animals, classification must be based on common basic features

Some other difficulties are:

(i) Tracing of interdependence amongst various animals will become difficult

(ii) Difficulty in developing new species of animals

2. If you are given a specimen, what are the steps that you would follow to classify it?


The steps to classify the specimen are:

(i) Classify level of organization: Classify the arrangement of cells in cellular and tissue level organization.

(ii) Symmetry: Classify the organism as radial or bilateral symmetry.

(iii) Classify Diploblastic or triploblastic organization

(iv) Presence or absence of body cavity

(v) Type of coelom development

(vi) Classify segmentation

(vii) Differentiate the presence or absence of notochord.

3. How useful is the study of the nature of the body cavity and coelom in the classification of animals?


The coelom is the body cavity or fluid-filled space lined by the mesoderm, and an animal with a coelom is called coelomate.

In some animals, the body cavity is not covered by the mesoderm; Instead, the mesoderm is a scattered cyst between the ectoderm and the endoderm.

Such a body cavity is called a pseudocolor and the animals that have them are called pseudocoelomates, for example – Aschelminthes. Some animals have no body cavity, they are called acoelomates, for example – Platyhelminthes.

Classification of the body cavity and coelom is important to decide the complexity of an organism at the organ level.

6. What are the peculiar features that you find in parasitic Platyhelminthes?


The typical features of the parasitic Platyhelminthes are:

(i) Free-living parasitic forms.

(ii) They have an organ level of organization.

(iii) Mostly hermaphrodites

(iv) Three-layered body wall – the epidermis (outer covering) is often ciliate and covered with cuticle.

(v) The Digestive tract is incomplete or absent

(vi) The presence of well-defined excretory structures, such as flame cells.

(vii) Presence of anti-toxins and a thick tegument which is resistant to the digestive enzymes of host. (viii) Anaerobic respiration. No special respiratory structure was observed.

(ix) The front body part has suckers, hooks, eyespots, and auricles to attach to the hosts.

(x) A highly developed reproductive system of parasitic forms.

7. What are the reasons that you can think of for the arthropods to constitute the largest group of the animal kingdom?


The following are the causes of arthropods, which make up the largest group of animal kingdoms

(i) They have jointed legs that allow them to motile, and perform many other functions due to these jointed appendages.

(ii) A hardened skeleton made of chitin protects the body.

(iii) Hard skeletons reduce water loss from the body.

(iv) Demonstrate a different system for locomotion, respiration, and reproduction.

(v) Ability to live in diverse conditions and varied habitats.

(vi) In comparison to other phyla, they are pre-developed.

(vii) Well-developed sense organs and nervous system.

(vii) Some insects exhibit pheromones that enable communication.

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