NCERT Solutions for Class 11 Biology Chapter 4: Animal Kingdom

Concept. More than a million species of animals have been described, and the need for classification becomes all the more important as the number grows. NCERT notes that, despite differences in structure and form, animals share fundamental features — arrangement of cells (level of organisation), body symmetry, nature of coelom, germ layers, segmentation, notochord, and patterns of digestive, circulatory or reproductive systems — and these form the basis of animal classification (Fig. 4.4). If we ignored such common fundamental features, we would face the following difficulties:

1. No uniform, reliable basis for grouping. With over a million species, any grouping based on superficial or arbitrary characters (size, colour, habitat, mode of movement) would be inconsistent. Different people would group the same animal differently.
2. Unrelated animals would be placed together. Superficial similarity is misleading: a whale lives in water like fishes, bats fly like birds, and a flying fish glides like a bird, yet whales and bats are mammals. Without the fundamental features (mammary glands, four-chambered heart, hair) they would be wrongly grouped with fishes and birds.
3. Related animals would be separated. Animals with the same body plan but different external appearance (for example, a sea star and a sea urchin — both radially symmetrical, coelomate echinoderms with water vascular systems) could be put in different groups.
4. Relationships and evolutionary trends could not be shown. The progression from cellular level (sponges) → tissue level (coelenterates) → organ level (flatworms) → organ-system level (annelids onward), from asymmetry → radial → bilateral symmetry, from acoelomate → pseudocoelomate → coelomate, would remain hidden.
5. No systematic position for newly described species. Classification “helps in assigning a systematic position to newly described species”; without fundamental features, a new species could not be slotted into a hierarchy.
6. Difficulty in identification, comparison and study. A student or scientist would have to study each animal separately, with no way of predicting characters of an unknown member of a group (for example, that every chordate has a notochord, dorsal hollow nerve cord and gill slits).
7. No common language between scientists. Without agreed fundamental criteria, the same group would be defined differently in different places, making communication, knowledge sharing and applied work (medicine, agriculture, conservation) unreliable.

Extra depth. Fundamental features are chosen because they are stable (do not change with the environment), constant within a group and show evolutionary advance, while superficial features (colour, size) vary widely even within a species.

Conclusion: without common fundamental features, the classification of over a million animal species would be arbitrary and inconsistent, would group unrelated forms and separate related ones, would hide evolutionary relationships, and would make it impossible to give a systematic position to a new species.

Concept. To classify an unknown specimen, observe its characters step by step, moving from the most general fundamental features to the specific distinctive features, exactly as in the broad classification of Fig. 4.4 and the salient features of Table 4.2.

1. Check that it is multicellular and heterotrophic — i.e., an animal (Kingdom Animalia: multicellular, eukaryotic, no cell wall).
2. Determine the level of organisation. Cellular (loose cell aggregates, pores — Porifera), tissue (Coelenterata, Ctenophora), organ (Platyhelminthes) or organ-system (all higher phyla).
3. Note the symmetry. Asymmetrical (sponges), radial (Coelenterata, Ctenophora, Echinodermata adults) or bilateral (rest).
4. Find the germinal layers. Diploblastic (ectoderm and endoderm with mesoglea) or triploblastic (with mesoderm).
5. Study the body cavity. Acoelomate (no cavity — flatworms), pseudocoelomate (cavity not lined by mesoderm — roundworms) or coelomate (true coelom lined by mesoderm).
6. Look for segmentation (metamerism — annelids, arthropods, chordates) and the type of digestive system (incomplete or complete) and circulatory system (open or closed).
7. Check for a notochord. If present → Chordata (also dorsal hollow nerve cord, paired gill slits, post-anal tail); if absent → a non-chordate phylum.
8. Match the distinctive features of the phylum (Table 4.2): pores and canals – Porifera; cnidoblasts – Cnidaria; comb plates – Ctenophora; flat body with suckers – Platyhelminthes; round worm-like – Aschelminthes; ring-like segments – Annelida; jointed appendages and chitinous exoskeleton – Arthropoda; shell, mantle, radula – Mollusca; spiny skin, water vascular system – Echinodermata; proboscis, collar, trunk – Hemichordata.
9. For a chordate, go further: jaws absent → Agnatha (Cyclostomata); jaws present → Gnathostomata; fins → Pisces (cartilaginous → Chondrichthyes; bony → Osteichthyes); limbs → Tetrapoda — moist skin, three-chambered heart → Amphibia; dry cornified skin → Reptilia; feathers and beak → Aves; hair and mammary glands → Mammalia.
10. Name it. Identify the genus and species and give the binomial name (as learnt in Chapter 1).

Conclusion: a specimen is classified by checking, in order, its level of organisation, symmetry, germ layers, coelom, segmentation and notochord, then matching the distinctive features with those of the phyla in Table 4.2, and finally assigning class, genus and species.

Concept. The body cavity is the space between the body wall and the gut wall. NCERT: “Presence or absence of a cavity between the body wall and the gut wall is very important in classification.” The cavity lined by mesoderm is called the coelom, and on this basis animals fall into three categories (Fig. 4.3):

Usefulness of the study of body cavity / coelom in classification:

1. It divides the triploblastic bilateral animals into three clear groups. In Fig. 4.4 the bilaterally symmetrical animals are split by body cavity into acoelomates (Platyhelminthes), pseudocoelomates (Aschelminthes) and coelomates (Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata) — a decisive step in the key.
2. It reflects the level of evolutionary advancement. Acoelomate → pseudocoelomate → coelomate shows increasing complexity; the true coelom is associated with the organ-system level of organisation.
3. It shows relationship with the germ layers. Only triploblastic animals (with a mesoderm) can have a coelom, since the coelom is lined by mesoderm; diploblastic animals cannot have a true coelom.
4. It helps to identify phyla and place unknown specimens. Whether the specimen has no cavity, a cavity not lined by mesoderm, or a mesoderm-lined cavity quickly narrows the possible phyla.
5. It is correlated with other features. Coelom is accompanied by well-developed organ systems, a complete digestive system, circulatory systems and segmentation (annelids, arthropods, chordates) — so one feature predicts many others.
6. It separates similar-looking animals. Roundworms and earthworms look alike (both “worms”) but roundworms are pseudocoelomate and earthworms coelomate, so they belong to different phyla.

Extra depth (standard biology). A true coelom lets the gut and body wall move independently, gives space for organs to grow and function, and provides a fluid-filled cushion — advantages that explain why most successful animal groups are coelomate.

Conclusion: the nature of the body cavity is a key criterion that separates bilateral triploblastic animals into acoelomates, pseudocoelomates and coelomates, indicates their evolutionary advancement and correlates with their organ-system organisation.

Concept. Digestion is the breakdown of complex food into simple absorbable substances by enzymes. It may take place inside the cells (intracellular) or outside the cells in a cavity or canal (extracellular).

Extra depth. In Cnidaria and Ctenophora both kinds occur together: partial digestion takes place extracellularly in the gastro-vascular cavity and the food particles are then completed intracellularly by the cells lining the cavity. In higher animals with a complete alimentary canal digestion is entirely extracellular.

Conclusion: intracellular digestion occurs inside food vacuoles of cells (e.g., sponges), whereas extracellular digestion occurs outside the cells in a cavity or canal (e.g., higher animals), with coelenterates and ctenophores showing both.

Concept. Development is the series of changes from the zygote to the adult. In some animals the young one that hatches or is born looks like a small adult; in others it passes through a larval stage that is very different from the adult. NCERT mentions this under the descriptions of each phylum.

Examples of larvae (standard). Tadpole of frog (Amphibia); veliger of molluscs; bipinnaria of starfish; caterpillar and maggot of insects; miracidium and cercaria of liver fluke.

Extra depth. Within arthropods and aschelminthes, development may be direct or indirect depending on the species (NCERT states this for both phyla).

Conclusion: in direct development the young resembles the adult (reptiles, birds, mammals, bony fishes), whereas in indirect development a morphologically distinct larva transforms into the adult by metamorphosis (sponges, flatworms, molluscs, echinoderms, amphibians).

Concept. Phylum Platyhelminthes (flatworms) includes free-living forms such as Planaria but is best known for its endoparasites living in animals, including human beings — Taenia (tape worm) and Fasciola (liver fluke). Parasites show special adaptations (peculiar features) to their parasitic mode of life.

Peculiar features of parasitic Platyhelminthes:

1. Dorso-ventrally flattened body — the body is leaf-like (liver fluke) or ribbon-like (tape worm), increasing surface area for absorption of food from the host.
2. Hooks and suckers — present in the parasitic forms; the scolex of the tape worm carries hooks and suckers, and the liver fluke has an oral and a ventral sucker; these attach the parasite firmly to the host’s tissues.
3. Absorption of nutrients through the body surface — some parasitic forms (e.g., tape worm) absorb nutrients directly from the host through their body surface; the digestive system is incomplete (single opening) or even absent.
4. Flame cells — specialised cells that help in osmoregulation and excretion (the flame cell/protonephridial system).
5. Bilaterally symmetrical, triploblastic, acoelomate with organ level of organisation — no body cavity; the space between gut and body wall is filled with parenchyma.
6. Sexes not separate (hermaphrodite) — each individual has both male and female reproductive organs, so one worm can reproduce (an advantage when the chance of meeting a mate in the host is small).
7. Internal fertilisation and development through many larval stages — the life cycle is complicated and involves larval stages and often an intermediate host (e.g., the liver fluke passes through a snail; the tape worm through a pig).
8. Very high reproductive capacity (standard fact) — large numbers of eggs are produced to compensate for the many hazards in transfer from one host to another.
9. Protective body covering and absence of locomotory organs (standard fact) — a tough cuticle/tegument protects against the host’s digestive enzymes and immune defences.

Extra depth. Non-parasitic flatworms such as Planaria have a high regeneration capacity; the parasitic forms trade away many free-living structures (sense organs, locomotory structures, alimentary canal in tape worm) for attachment and reproduction.

Conclusion: parasitic flatworms are flattened, acoelomate animals equipped with hooks and suckers for attachment, direct absorption of nutrients through the body surface, flame cells for excretion, hermaphrodite reproduction and a life cycle with many larval stages.

Concept. Arthropoda is the largest phylum of Animalia, and over two-thirds of all named species on earth are arthropods (insects, crustaceans, arachnids, myriapods). Their success comes from a combination of structural and functional features that made them well adapted to almost every habitat.

Reasons for their success:

1. Chitinous exoskeleton — a tough, waterproof cuticle protects the body from mechanical injury, predators and desiccation, allowing life on land as well as in water; it also gives support for muscle attachment.
2. Jointed appendages (arthros = joint, poda = appendages) — legs, mouthparts, antennae modified for walking, running, jumping, swimming, flying, feeding, sensing and mating; this gives enormous versatility in locomotion and feeding.
3. Segmented body with tagmatisation — the body is divided into head, thorax and abdomen, with division of labour (sensory and feeding in head, locomotion in thorax, digestion and reproduction in abdomen).
4. Efficient respiratory organs — gills (aquatic), book gills, book lungs or the tracheal system (insects) deliver oxygen directly to tissues, supporting high activity on land.
5. Wings in insects — flight helps escape predators, find food and mates, and disperse to new habitats; insects are the only invertebrates that fly.
6. Well-developed sense organs — antennae, compound and simple eyes, statocysts (balancing organs) give quick response to the environment.
7. Efficient excretion and water conservation — Malpighian tubules excrete nitrogenous waste with minimal water loss; open circulation is adequate for small bodies.
8. Reproductive success — mostly dioecious with usually internal fertilisation; mostly oviparous with large numbers of eggs and short life cycles, so populations grow rapidly.
9. Direct or indirect development — in indirect development (metamorphosis) larva and adult occupy different habitats and use different food (e.g., caterpillar vs butterfly), which reduces competition between the young and adult stages.
10. Small size and adaptability — small bodies need little food and space, so very many individuals and species can live in a habitat; arthropods occupy aquatic, terrestrial, aerial, parasitic and symbiotic niches.

Conclusion: arthropods are the largest group because their chitinous exoskeleton, jointed appendages, segmented head-thorax-abdomen body, efficient respiratory and excretory systems, flight (in insects), high reproductive capacity and metamorphosis let them exploit almost every habitat.

Concept. The water vascular (ambulacral) system is a system of fluid-filled canals and tube feet; NCERT states that “the most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration.”

Answer: (c) Echinodermata.

Structure and function (standard facts). In a starfish, sea water enters through a sieve-like plate, the madreporite (on the upper surface), passes via the stone canal into a ring canal around the mouth and then into radial canals running along each arm. Short lateral canals end in muscular tube feet (podia) with sucker-like tips. Contraction of the bulb-like ampulla pushes fluid into the tube foot, which extends and sticks to the substratum; fluid pressure changes make the tube feet move in a coordinated way. Functions: locomotion, capture and transport of food (e.g., opening bivalves) and respiration (gas exchange through thin tube-foot walls).

Extra depth. Other echinoderm features: endoskeleton of calcareous ossicles (hence spiny-bodied), complete digestive system with mouth on the ventral side and anus on the dorsal side, no excretory system, separate sexes, external fertilisation, indirect development with a free-swimming bilaterally symmetrical larva. Examples: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).

Conclusion: the water vascular system is the characteristic feature of phylum Echinodermata (option c), helping in locomotion, capture and transport of food and respiration.

Concept. Phylum Chordata includes all animals that possess, at some stage of life, a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits; they also have a post-anal tail and a closed circulatory system (Fig. 4.16). The phylum is divided into three subphyla: Urochordata (Tunicata), Cephalochordata and Vertebrata. Vertebrates are only one of these subphyla.

Why all vertebrates are chordates. The members of subphylum Vertebrata possess the notochord during the embryonic period, and also show the dorsal hollow nerve cord, gill slits (at least in the embryo) and a post-anal tail. In the adult the notochord is replaced by a cartilaginous or bony vertebral column. Since they have all the basic chordate characters, every vertebrate is a chordate. Besides these, they have a ventral muscular heart with two, three or four chambers, kidneys and paired appendages (fins or limbs).

Why not all chordates are vertebrates. The two other subphyla — Urochordata and Cephalochordata (together called protochordates, exclusively marine) — are chordates but have no vertebral column:

• Urochordata (Tunicata) — the notochord is present only in the larval tail and is lost in the adult (e.g., Ascidia, Salpa, Doliolum).
• Cephalochordata — the notochord extends from head to tail and is persistent throughout life, but it is never replaced by a vertebral column (e.g., Branchiostoma = Amphioxus or lancelet).

Extra depth — classification of Vertebrata (NCERT). Vertebrata → Agnatha (jawless): class Cyclostomata; Gnathostomata (jawed): super class Pisces (fins; classes Chondrichthyes, Osteichthyes) and super class Tetrapoda (limbs; classes Amphibia, Reptilia, Aves, Mammalia). Remember also Table 4.1: chordates have a dorsal, hollow, single nerve cord, a ventral heart and gill slits, while non-chordates have a ventral, solid, double nerve cord and a dorsal heart (if present).

Conclusion: all vertebrates are chordates because they show the chordate characters, but the protochordates (Urochordata and Cephalochordata) are chordates without a vertebral column, so not all chordates are vertebrates.

Concept. Pisces (super class of fishes bearing fins) includes two classes: Chondrichthyes (cartilaginous fishes) and Osteichthyes (bony fishes). The air bladder is a thin-walled, gas-filled sac lying in the body cavity above the gut of bony fishes. NCERT states that in Osteichthyes the “air bladder is present which regulates buoyancy”, whereas in Chondrichthyes, “due to the absence of air bladder, they have to swim constantly to avoid sinking.”

Importance of the air bladder:

1. Regulates buoyancy. By changing the volume of gas in the bladder (gas is secreted into or absorbed from it), a bony fish adjusts its overall density to equal that of the surrounding water, so it can float or hover at any desired depth without effort.
2. Saves energy. Since the fish does not have to keep swimming to avoid sinking, it spends far less energy — contrast with cartilaginous fishes such as Scoliodon (dog fish), which must swim constantly because they lack an air bladder.
3. Permits slow, precise movements and stationary life. Fishes like Labeo (rohu) and Hippocampus (sea horse) can stay motionless, feed in mid-water or rest.
4. Allows depth adjustment for feeding, migration and escape (standard fact) — the fish can move to different levels in the water with little muscular effort.
5. Other secondary functions in some fishes (standard facts) — as an accessory respiratory organ in some air-breathing fishes (lungfishes), for sound production and hearing (connected to the inner ear in some).

Extra depth. Cartilaginous fishes compensate for the lack of an air bladder with oily livers (low density), a streamlined body and broad pectoral fins and a tail that give lift while swimming — but they still sink when they stop.

Conclusion: the air bladder in Pisces (bony fishes) regulates buoyancy, letting the fish stay at any depth without constant swimming and thereby saving energy; cartilaginous fishes, which lack it, must swim all the time to avoid sinking.

Concept. Aves (birds) are characterised by the presence of feathers, and most of them can fly (except flightless birds like the ostrich). Flight needs a body that is light, streamlined, strong, powered by an efficient respiratory and circulatory system and supplied with plenty of energy. Birds show the following modifications (NCERT features, with the standard supporting facts):

1. Feathers. The body is covered with feathers; contour and flight feathers form the broad light surface of the wings and tail for lift, propulsion and steering, and the body feathers streamline the body and insulate it.
2. Forelimbs modified into wings. The forelimbs are modified into wings, the main organs of flight, which provide the lifting and propelling surface.
3. Beak. The jaws form a light horny beak, replacing heavy teeth and jaw muscles, so the head is light (beak shape varies with food: parrot, vulture, etc.).
4. Hind limbs. The hind limbs generally have scales and are modified for walking, swimming or clasping tree branches; with the forelimbs freed for flying, the hind limbs carry out landing, perching and walking.
5. Dry skin without glands except the oil gland. The skin is dry and light; the only gland is the oil gland at the base of the tail, whose oil waterproofs and preens the feathers.
6. Pneumatic (hollow) bones. The endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic), which greatly reduces body weight while keeping strength.
7. Digestive tract with crop and gizzard. The digestive tract has additional chambers — the crop (stores food) and the gizzard (grinds food, replacing teeth) — so food is processed quickly and weight is kept near the centre of gravity.
8. Completely four-chambered heart. The heart is completely four-chambered, so oxygenated and deoxygenated blood do not mix, supplying the enormous oxygen needs of flight muscles.
9. Warm-blooded (homoiothermous). Birds maintain a constant body temperature, so they remain active in different weather and at different heights and temperatures.
10. Efficient respiration with air sacs. Respiration is by lungs, and air sacs connected to the lungs supplement respiration (providing a continuous supply of fresh air and reducing body density).
11. Other standard adaptations. Streamlined, spindle-shaped body that reduces air resistance; a keeled breast bone (sternum) for attachment of powerful flight muscles; fused bones (e.g., synsacrum) for a rigid skeleton; and a short tail (pygostyle) bearing tail feathers that act as a rudder and brake.

Extra depth. Flightless birds such as the ostrich and penguin show how these features are modified when flight is lost — for instance penguins have flippers for swimming and ostriches have strong hind limbs for running.

Conclusion: birds fly with the help of feathers, wings formed from modified forelimbs, a beak instead of teeth, light pneumatic bones, a crop and gizzard, a completely four-chambered heart, lungs with air sacs and warm-bloodedness, all of which make the body light, strong, streamlined and well supplied with energy.

Concept. In oviparous animals the female lays eggs and the embryo develops outside the mother’s body (e.g., amphibians, most fishes, reptiles, birds, arthropods, and the platypus); in viviparous animals the zygote develops inside the mother’s body and the female gives birth to young ones (most mammals, e.g., humans, dogs, kangaroo; and some sharks).

Answer: No — the number is not equal. An oviparous mother produces a very large number of eggs, whereas a viviparous mother produces only a few young ones at a time. The reasons are:

1. Chances of survival. In oviparous animals the eggs (and young) are exposed to predators, parasites, desiccation, temperature changes and other environmental hazards; many do not survive, so a large number of eggs must be produced to ensure that at least some reach adulthood. In viviparous animals the embryo develops inside the mother’s body, protected and nourished, so survival chances are high and few young ones are enough.
2. Nourishment and protection of the embryo. Oviparous eggs have to carry all the food (yolk) they need and little parental care is given in fishes and amphibians; in viviparous mammals the embryo is nourished through the placenta and later by milk (mammary glands).
3. Energy and body space. A viviparous mother must carry developing young inside her body and supply them with nutrients and oxygen, which limits the number she can bear at one time; an oviparous mother only forms and lays the eggs.
4. Parental care. Viviparous animals (mammals) give extensive parental care after birth, so a few young are enough; oviparous animals with external fertilisation (fishes, amphibians) give little or no care and compensate with numbers.
5. Type of fertilisation. External fertilisation (Osteichthyes, Amphibia) is chancy — many eggs and sperms are wasted — so large numbers of gametes are produced, while internal fertilisation with viviparity is far more efficient.

Examples. A bony fish such as Catla or the frog lays thousands of eggs, a hen lays a few eggs in a clutch, while a human mother usually bears one child, a cat or dog a few puppies/kittens and an elephant only one.

Extra depth. The inverse relationship — more parental care and internal development, fewer offspring — is a general reproductive strategy in animals. NCERT notes that Chondrichthyes have internal fertilisation and many are viviparous, and that most mammals are viviparous “with few exceptions” (the platypus lays eggs).

Conclusion: the numbers cannot be equal: an oviparous mother lays many eggs to offset losses outside the body, while a viviparous mother, whose embryos are protected and nourished inside her body and cared for after birth, produces only a few young ones.

Concept. Segmentation (metamerism) is the condition in which the body is externally and internally divided into segments with a serial repetition of at least some organs (e.g., in the earthworm). NCERT: “the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.”

Answer: (c) Annelida.

Explanation. In the animal kingdom sequence of phyla (Fig. 4.4 and Table 4.2), the lower phyla — Porifera, Coelenterata, Ctenophora, Platyhelminthes and Aschelminthes — show no segmentation. Annelids are the first animals to be metamerically segmented and also the first with a true coelom; they have longitudinal and circular muscles, a closed circulatory system, nephridia and a double ventral nerve cord, all repeated segment by segment. Arthropods (and chordates) show segmentation later in the sequence; Mollusca, Echinodermata and Hemichordata, though coelomate, are unsegmented.

Conclusion: segmentation is first observed in phylum Annelida (option c), whose body is divided into ring-like metameres with serial repetition of organs.

Concept. Each structure listed in column I is a characteristic feature of one animal group in column II (NCERT chapter descriptions).

Final answer: (a)–(viii), (b)–(v), (c)–(iv), (d)–(i), (e)–(ii), (f)–(vii), (g)–(iii), (h)–(vi).

Extra depth. Notice the overlaps that make matching tricky: scales also occur in fishes (placoid in Chondrichthyes, cycloid/ctenoid in Osteichthyes), and gill slits occur in all fishes and in the embryos of higher chordates — but among the options given, scales best match reptiles with their epidermal scales, and “gill slits” best match the group having separate gill slits without operculum (Cyclostomata and Chondrichthyes), the only unmatched option left.

Conclusion: the correct matches are operculum – Osteichthyes, parapodia – Annelida, scales – Reptilia, comb plates – Ctenophora, radula – Mollusca, hairs – Mammalia, choanocytes – Porifera and gill slits – Cyclostomata and Chondrichthyes.

Concept. A parasite lives on (ectoparasite) or inside (endoparasite) another organism, the host, and obtains food from it, harming it. Many animal phyla include parasites of humans. Based on the NCERT chapters, the following animals are found parasitic on human beings:

Note. Anopheles, Culex and Aedes mosquitoes are vectors (carriers) of disease agents rather than true parasites living on human beings; NCERT lists them as arthropod vectors.

Extra depth. Parasitic flatworms have hooks and suckers; roundworms are dioecious with a complete alimentary canal; cleanliness, safe drinking water, cooked meat and vector control are the main methods of prevention.

Conclusion: animals parasitic on humans include the protozoans Entamoeba, Plasmodium and Trypanosoma, the flatworms Taenia and Fasciola, the roundworms Ascaris, Wuchereria and Ancylostoma, the leech Hirudinaria and ectoparasitic arthropods such as lice and mites.