NCERT Solutions for Class 11 Biology Chapter 4: Animal Kingdom (NCERT 2026–27)

These Class 11 Biology Chapter 4 solutions cover Animal Kingdom from the NCERT textbook (session 2026–27). Every numbered question in the end-of-chapter Exercises is reproduced exactly as printed and answered in clear, exam-ready prose. The chapter explains how over a million described animal species are classified using a few fundamental features — levels of organisation, body symmetry, germ layers, coelom, segmentation and the notochord — and then surveys the salient characters of every major phylum from Porifera to Chordata.

Class: 11 Subject: Biology Chapter: 4 Chapter Name: Animal Kingdom Exercises: 15 questions Session: 2026–27
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Class 11 Biology Chapter 4 Solutions – Overview

Animal Kingdom (Animalia) contains more than a million described species, so a logical system of classification is essential both for study and for placing newly discovered organisms. Despite their enormous diversity, animals share a small set of fundamental features that form the basis of classification: the level of organisation (cellular, tissue, organ, organ-system), the type of symmetry (asymmetrical, radial, bilateral), the number of germ layers (diploblastic or triploblastic), the nature of the coelom (coelomate, pseudocoelomate, acoelomate), the presence of segmentation and of a notochord. Using these criteria, Animalia is divided into major phyla — Porifera, Coelenterata (Cnidaria), Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata. Chordates are further split into Urochordata, Cephalochordata and Vertebrata, and vertebrates into the classes Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia.

Key Concepts & Definitions

Levels of organisation: cellular (Porifera), tissue (Coelenterata, Ctenophora), organ (some Platyhelminthes) and organ-system (Aschelminthes onwards) levels of body organisation.

Symmetry: asymmetrical (most sponges), radial (Coelenterata, Ctenophora, adult Echinodermata) where any plane through the central axis gives identical halves, and bilateral (Platyhelminthes onwards) where only one plane gives identical left and right halves.

Diploblastic vs triploblastic: diploblastic animals have two germ layers (ectoderm and endoderm) with mesoglea in between (Coelenterata); triploblastic animals have a third layer, mesoderm, between them (Platyhelminthes to Chordates).

Coelom: a body cavity lined by mesoderm. Coelomates have a true coelom (Annelida onwards); pseudocoelomates have a body cavity not lined by mesoderm (Aschelminthes); acoelomates have no body cavity (Platyhelminthes).

Segmentation (metamerism): serial repetition of body segments with at least some organs repeated, e.g. the metameres of an earthworm.

Notochord: a mesodermally derived, dorsal rod-like structure formed during embryonic development; animals that form it are chordates, those that do not are non-chordates (Porifera to Echinodermata).

Other key terms: choanocytes (collar cells of sponges), cnidoblasts/nematocysts (stinging cells of cnidarians), flame cells (excretion in flatworms), nephridia (excretion in annelids), malpighian tubules (excretion in arthropods), radula (rasping organ of molluscs) and the water vascular system (locomotion in echinoderms).

NCERT Exercises — Full Solutions

All questions below are reproduced verbatim from the NCERT textbook (Reprint 2026–27). Answers are original and written in CBSE exam-ready style.

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

ANSWER If the common fundamental features were ignored, classification would have to rely only on superficial likeness, which would create serious problems: • Animals that merely look alike but are basically different (for example, a whale and a fish) might be grouped together, while truly related animals could be separated. • With more than a million species, there would be no logical, shared criteria, so the system would become chaotic and almost impossible to remember or use. • Newly discovered species could not be assigned a definite systematic position, because there would be no fixed characters (level of organisation, symmetry, coelom, etc.) to compare them with. • Evolutionary relationships would be lost, making it impossible to understand how different groups are connected. Therefore, the fundamental features give us a stable, comparable and meaningful basis for classifying animals.

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

ANSWER To classify a given specimen, I would examine its fundamental features one by one, in this order: • Level of organisation: are cells loose aggregates (cellular), or organised into tissues, organs or organ-systems? • Symmetry: is the body asymmetrical, radially symmetrical or bilaterally symmetrical? • Germ layers: is it diploblastic (two layers) or triploblastic (three layers)? • Coelom: is the animal acoelomate, pseudocoelomate or coelomate? • Segmentation: is the body metamerically segmented or not? • Notochord: is a notochord present (chordate) or absent (non-chordate)? Along with these, I would note distinctive features such as the type of digestive, circulatory and respiratory systems, mode of reproduction and any special structures. Matching these characters with the known features of the phyla finally places the specimen in its correct group.

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

ANSWER The nature of the body cavity is a very important and reliable basis of classification because it cleanly separates the triploblastic animals into three grades: • Coelomates — animals with a true coelom lined by mesoderm, e.g. Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata and Chordata. • Pseudocoelomates — animals whose body cavity is not lined by mesoderm; instead the mesoderm lies as scattered pouches between ectoderm and endoderm, e.g. Aschelminthes. • Acoelomates — animals with no body cavity between the body wall and the gut, e.g. Platyhelminthes. Because the type of coelom is a deep, internal and constant feature, it helps assign animals to the correct phylum and also reflects increasing complexity, making it far more dependable than external appearance.

4. Distinguish between intracellular and extracellular digestion?

ANSWER The two types of digestion differ mainly in where food is broken down:
Intracellular digestionExtracellular digestion
Digestion takes place inside the cells (within food vacuoles).Digestion takes place outside the cells, in a digestive cavity.
It is a primitive method found in simpler animals.It is a more advanced method found in higher animals.
Food is first engulfed by cells and then digested by enzymes within them.Enzymes are secreted into the cavity, where food is digested and then absorbed.
Example: Porifera (sponges).Example: most higher animals; in Coelenterata and Ctenophora digestion is both extracellular and intracellular.

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

ANSWER Direct development: the young one that hatches or is born already resembles the adult in form. It simply grows in size to become an adult, and there is no larval stage. Examples include roundworms (in many cases), reptiles, birds and mammals. Indirect development: the egg gives rise to a larva that is morphologically distinct from the adult. The larva later undergoes metamorphosis to become the adult. Examples include sponges, many insects, amphibians (frog) and echinoderms.

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

ANSWER Parasitic platyhelminthes (flatworms such as Taenia and Fasciola) show several special adaptations to a parasitic life: • They possess hooks and suckers for firm attachment to the host’s body. • Many of them have no functional digestive system and instead absorb digested nutrients directly through their body surface from the host. • The body is dorso-ventrally flattened, increasing the surface area for absorption. • They have a tough body covering (tegument) that resists the host’s digestive enzymes. • They show high reproductive capacity, producing large numbers of eggs and passing through several larval stages, which improves the chance of finding a new host.

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

ANSWER Arthropods are the most successful and largest group (over two-thirds of all named species) because of several adaptive features: • A hard, protective chitinous exoskeleton that prevents water loss and supports life on land. • Jointed appendages that allow efficient and varied movement (walking, swimming, flying). • A body divided into head, thorax and abdomen with division of labour. • Well-developed sensory organs (antennae, compound eyes, statocysts) for sensing the environment. • Varied respiratory organs (gills, book gills, book lungs, tracheal system) suiting many habitats. • High reproductive rate, mostly internal fertilisation and the ability to occupy almost every habitat on earth. Together these adaptations have allowed arthropods to flourish in enormous numbers.

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

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

ANSWER (c) Echinodermata. The water vascular system is the most distinctive feature of echinoderms (e.g. Asterias, the star fish). It is a network of fluid-filled canals and tube feet that helps in locomotion, capture and transport of food, and respiration.

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

ANSWER All chordates possess the three basic chordate characters — a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits — at some stage of life. Vertebrates also possess all these characters, so every vertebrate is a chordate. However, the phylum Chordata also includes the subphyla Urochordata and Cephalochordata (the protochordates), in which the notochord is not replaced by a vertebral column. In vertebrates, the notochord present during the embryonic period is later replaced by a cartilaginous or bony vertebral column in the adult. Since protochordates are chordates but lack a vertebral column, all chordates are not vertebrates. Hence the statement is justified.

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

ANSWER The air bladder (swim bladder) is present in bony fishes (Osteichthyes) and is very important for regulating buoyancy. By adjusting the gas in the bladder, the fish can control its position and float at a desired depth in water without continuous effort. Its importance is clear from the contrast with cartilaginous fishes (Chondrichthyes), which lack an air bladder; because of this, they must swim constantly to avoid sinking. Thus the air bladder saves energy and gives bony fishes better control over their movement in water.

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

ANSWER Birds (Aves) show many modifications (flight adaptations): • The body is covered with feathers, and the forelimbs are modified into wings for flight. • The endoskeleton is fully ossified and the long bones are hollow with air cavities (pneumatic), making the body light. • Air sacs connected to the lungs supplement respiration and provide extra oxygen during flight, also reducing weight. • The heart is completely four-chambered and they are warm-blooded, ensuring efficient supply of oxygen and energy. • The presence of a crop and gizzard in the digestive tract and a streamlined body further help in efficient flight.

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

ANSWER No, the numbers are generally not equal. An oviparous mother lays eggs in the external environment, where they face many dangers such as predators, drying and temperature changes, so the chance of survival is low. To make sure that at least a few survive, oviparous animals usually produce a large number of eggs. A viviparous mother gives birth to young ones that have developed and been protected inside her body, so their chance of survival is much higher. Therefore viviparous animals produce far fewer young ones. Because the survival rate differs so greatly, the numbers cannot be equal.

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

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

ANSWER (c) Annelida. True metameric segmentation is first observed in Annelida, where the body is externally and internally divided into segments (metameres) with serial repetition of organs, e.g. the earthworm Pheretima.

14. Match the following:

ANSWER The correct matches are given below.
Column IColumn II
(a) Operculum(viii) Osteichthyes
(b) Parapodia(v) Annelida
(c) Scales(iv) Reptilia
(d) Comb plates(i) Ctenophora
(e) Radula(ii) Mollusca
(f) Hairs(vii) Mammalia
(g) Choanocytes(iii) Porifera
(h) Gill slits(vi) Cyclostomata and Chondrichthyes

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

ANSWER Some animals that are parasitic on human beings include: • Taenia (Tapeworm) — intestinal parasite. • Fasciola (Liver fluke) — parasite of the liver/bile passages. • Ascaris (Roundworm) — intestinal parasite. • Wuchereria (Filaria worm) — causes filariasis (elephantiasis). • Ancylostoma (Hookworm) — intestinal blood-sucking parasite. • Plasmodium (malarial parasite, transmitted by the mosquito vector Anopheles). • Ectoparasites such as the head louse (Pediculus) and the leech (Hirudinaria), which sucks blood.

Extra Practice Questions

Short Answer Type Questions

Q1. Define coelom. Name one coelomate and one acoelomate animal.

ANSWERThe coelom is a body cavity between the body wall and the gut that is lined by mesoderm. A coelomate example is Annelida (earthworm); an acoelomate example is Platyhelminthes (tapeworm).

Q2. What are cnidoblasts? State their functions.

ANSWERCnidoblasts (cnidocytes) are stinging cells found on the tentacles and body of cnidarians; they contain the stinging capsules called nematocysts. They are used for anchorage, defence and capture of prey.

Q3. Differentiate between polyp and medusa.

ANSWERA polyp is a sessile, cylindrical body form (e.g. Hydra, Adamsia), while a medusa is an umbrella-shaped, free-swimming form (e.g. Aurelia, jellyfish). In some cnidarians like Obelia both forms occur, showing alternation of generations (metagenesis).

Q4. Why are echinoderm larvae important in classification despite the adults being radially symmetrical?

ANSWERAdult echinoderms are radially symmetrical, but their larvae are bilaterally symmetrical. This larval bilateral symmetry suggests that echinoderms evolved from bilaterally symmetrical ancestors and links them to the bilaterian line that leads towards chordates, which is significant for understanding their relationships.

Q5. List the three subphyla of Chordata with one example of each.

ANSWERUrochordata — Ascidia; Cephalochordata — Branchiostoma (Amphioxus); Vertebrata — any vertebrate such as Rana (frog).

Long Answer Type Questions

Q1. Describe the salient features of the phylum Arthropoda with examples.

ANSWERArthropoda is the largest phylum of Animalia. Members have an organ-system level of organisation and are bilaterally symmetrical, triploblastic, segmented and coelomate. The body is covered by a chitinous exoskeleton and consists of head, thorax and abdomen with jointed appendages. Respiration occurs through gills, book gills, book lungs or a tracheal system, and the circulatory system is of the open type. Sensory organs include antennae, compound and simple eyes and statocysts; excretion is by malpighian tubules. They are mostly dioecious and oviparous with internal fertilisation, and development may be direct or indirect. Examples: Apis (honey bee), Bombyx (silkworm), Anopheles (mosquito), Locusta (locust) and Limulus (king crab, a living fossil).

Q2. Compare the characters of chordates and non-chordates.

ANSWERChordates and non-chordates differ in several key features as summarised below.
ChordatesNon-chordates
Notochord present.Notochord absent.
Central nervous system is dorsal, hollow and single.Central nervous system is ventral, solid and double.
Pharynx perforated by gill slits.Gill slits absent.
Heart is ventral.Heart is dorsal (if present).
A post-anal part (tail) is present.Post-anal tail absent.

Q3. Distinguish among the three classes of fishes and amphibians on the basis of heart chambers, skeleton and habitat.

ANSWERChondrichthyes are marine cartilaginous fishes with a two-chambered heart and persistent notochord; lacking an air bladder, they swim constantly. Osteichthyes are marine and freshwater bony fishes with a two-chambered heart, a bony endoskeleton and an air bladder that regulates buoyancy. Amphibians can live both in water and on land; they have a three-chambered heart (two auricles, one ventricle), moist scaleless skin, and respire through gills, lungs and skin. All three groups are cold-blooded (poikilothermous), but they differ in skeleton, heart chambers and the range of habitats they occupy.

MCQs

1. Cellular level of organisation is exhibited by:

(a) Coelenterata    (b) Porifera    (c) Annelida    (d) Arthropoda

2. The body cavity lined by mesoderm is called:

(a) pseudocoelom    (b) blastocoel    (c) coelom    (d) gastro-vascular cavity

3. Flame cells for excretion are found in:

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

4. Malpighian tubules are the excretory organs of:

(a) Annelida    (b) Echinodermata    (c) Arthropoda    (d) Porifera

5. The radula is a feeding organ characteristic of:

(a) Mollusca    (b) Annelida    (c) Coelenterata    (d) Aschelminthes

6. Which phylum is pseudocoelomate?

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

7. Comb plates for locomotion are present in:

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

8. Which class of vertebrates is jawless (Agnatha)?

(a) Chondrichthyes    (b) Osteichthyes    (c) Cyclostomata    (d) Amphibia

9. A four-chambered heart is found in:

(a) Amphibia    (b) all reptiles    (c) Aves and Mammalia    (d) Chondrichthyes

10. Ornithorhynchus (Platypus) is an example of:

(a) an oviparous mammal    (b) a viviparous reptile    (c) a bird    (d) an amphibian

Answer key: 1-(b), 2-(c), 3-(b), 4-(c), 5-(a), 6-(b), 7-(b), 8-(c), 9-(c), 10-(a).

Assertion–Reason Questions

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Cartilaginous fishes have to swim constantly to avoid sinking.

Reason: They lack an air bladder to regulate buoyancy.

A-R 2. Assertion: All chordates are vertebrates.

Reason: All chordates possess a notochord at some stage of life.

A-R 3. Assertion: Adult echinoderms are radially symmetrical.

Reason: Echinoderm larvae are also radially symmetrical.

A-R 4. Assertion: Oviparous animals usually produce more eggs than the young ones born to viviparous animals.

Reason: Eggs laid in the external environment face greater danger, so a large number ensures survival of a few.

A-R 5. Assertion: Annelids are metamerically segmented animals.

Reason: Their body is divided into segments with serial repetition of at least some organs.

Answer key: 1-(A), 2-(D), 3-(C), 4-(A), 5-(A).

Common Mistakes & Exam Tips

Common mistakes to avoid

  • Confusing coelom, pseudocoelom and acoelom — remember only the true coelom is fully lined by mesoderm.
  • Writing that all chordates are vertebrates — protochordates (Urochordata, Cephalochordata) are chordates but not vertebrates.
  • Mixing up excretory organs: flame cells (Platyhelminthes), nephridia (Annelida), malpighian tubules (Arthropoda).
  • Forgetting that true segmentation first appears in Annelida, not in flatworms or roundworms.
  • Stating reptiles have a four-chambered heart — it is three-chambered except in crocodiles.
  • Mis-spelling scientific names — always write them in italics with the genus capitalised (e.g. Pheretima).

How to score full marks in this chapter

Learn the hierarchy of fundamental features (organisation → symmetry → germ layers → coelom → segmentation → notochord) and use it to classify any specimen. Memorise the salient-features table (Table 4.2) with at least two examples per phylum, and revise the chordate-vs-non-chordate comparison. For one-word and matching questions, keep a quick list pairing each structure with its phylum (operculum–Osteichthyes, parapodia–Annelida, radula–Mollusca, choanocytes–Porifera). Write scientific names correctly and support every classification statement with an example.

Frequently Asked Questions

What is Class 11 Biology Chapter 4 Animal Kingdom about?

Chapter 4, Animal Kingdom, explains how animals are classified using fundamental features such as level of organisation, symmetry, germ layers, coelom, segmentation and the notochord, and then describes the salient features of every major phylum from Porifera to Chordata, including the vertebrate classes.

How many questions are there in the NCERT exercise of this chapter?

The end-of-chapter Exercises section has 15 numbered questions, including two multiple-choice questions and one matching question. All of them are reproduced verbatim and fully solved on this page.

Why is the study of coelom important in classification?

The nature of the body cavity divides triploblastic animals into coelomates, pseudocoelomates and acoelomates. Because it is a deep internal feature, it is far more reliable than external appearance and reflects increasing complexity, making it a key tool in classifying animals.

Are these Class 11 Biology Chapter 4 solutions free?

Yes. All solutions are free and follow the official NCERT Biology textbook for session 2026-27.

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