NCERT Book Class 11 Biology Animal Kingdom

ANIMAL KINGDOM

4.1 BASIS OF CLASSIFICATION

Inspite of 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, 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 cellular level of organisation. Some division of labour (activities) occur 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 organisation. A still higher level of organisation, i.e.,  organ level is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised for a particular function. In animals like Annelids, Arthropods, Molluscs,

Echinoderms and Chordates, organs have associated to form functional systems, each system concerned with a specific physiological function. This pattern is called organ system level of organisation. 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. A complete digestive system has two openings, mouth and anus. Similarly, the circulatory system may be of two types:

(i) open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it or 

(ii) closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).

4.1.2 Symmetry

Animals can be categorised on the basis of their symmetry. Sponges are mostly asymmetrical, i.e., any plane that passes through the centre 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 (Figure 4.1b).

EXERCISES

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

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

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

4. Distinguish between intracellular and extracellular digestion?

5. What is the difference between direct and indirect development?

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

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

8. Water vascular system is the characteristic of which group of the following:

(a) Porifera (b) Ctenophora (c) Echinodermata (d) Chordata

9. “All vertebrates are chordates but all chordates are not vertebrates”. Justify the statement.

10. How important is the presence of air bladder in Pisces?

11. What are the modifications that are observed in birds that help them fly?

12. Could the number of eggs or young ones produced by an oviparous and viviparous mother be equal? Why?

13. Segmentation in the body is first observed in which of the following:

(a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthropoda

14. Match the following:

(i) Operculum              (a) Ctenophora

(ii) Parapodia               (b) Mollusca

(iii) Scales                   (c) Porifera

(iv) Comb plates           (d) Reptilia

(v) Radula                    (e) Annelida

(vi) Hairs                      (f) Cyclostomata and Chondrichthyes

(vii) Choanocytes          (g) Mammalia

(viii) Gill slits                 (h) Osteichthyes

15. Prepare a list of some animals that are found parasitic on human beings.

Please refer to attached file for NCERT Class 11 Biology -  Animal Kingdom