Animal Kingdom Class 11 Notes

Animals are classified based on several fundamental features of their body plan. These criteria form the foundation for placing organisms into the correct phylum.

• Segmentation — the body is divided into repeated units (segments or metameres). Examples: earthworm (Annelida), cockroach (Arthropoda). In vertebrates, segmentation is seen in the vertebral column and spinal nerves.
• Notochord — a rod-like structure derived from mesoderm, found in chordates. It provides skeletal support and is the defining feature of Phylum Chordata. In vertebrates, it is replaced by the vertebral column during development.

• Arthropods show the most successful animal body plan — they occupy every habitat.
• They respire by gills (aquatic forms), book lungs (scorpions), tracheal system (insects) or book gills (king crab).
• Insects are the largest class within Arthropoda.

• Urochordata (tunicates) — notochord present only in the larva; adults are sessile and filter feeders. Example: Ascidia, Salpa.
• Cephalochordata — notochord persistent throughout life; the body is fish-like. Example: Branchiostoma (Amphioxus).
• Vertebrata — the notochord is replaced by a vertebral column of vertebrae. They have a well-developed head, brain, and sensory organs. Further classified into jawless (Agnatha — e.g. Petromyzon/lamprey, Myxine/hagfish) and jawed (Gnathostomata — Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia).

Example: An animal is triploblastic, acoelomate, bilaterally symmetric and parasitic. Identify its phylum and give two examples.

Solution: The animal belongs to Phylum Platyhelminthes (flatworms). Key identifiers: triploblastic + acoelomate + bilateral symmetry. Examples include Taenia solium (tapeworm, parasitic in human intestine) and Fasciola hepatica (liver fluke, parasitic in sheep liver).

Example: Why is Arthropoda the largest phylum of the animal kingdom? List four features that contribute to its success.

Solution: Arthropoda is the largest phylum (over 10 lakh described species) because of: (1) jointed appendages for diverse locomotion — walking, swimming, flying; (2) chitinous exoskeleton providing protection and preventing water loss; (3) segmental body plan allowing regional specialisation; and (4) ability to occupy almost every habitat — terrestrial, aquatic and aerial.

Exercises (NCERT Q1–Q15)

• 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: (a) Operculum (i) Ctenophora (b) Parapodia (ii) Mollusca (c) Scales (iii) Porifera (d) Comb plates (iv) Reptilia (e) Radula (v) Annelida (f ) Hairs (vi) Cyclostomata and Chondrichthyes (g) Choanocytes (vii) Mammalia (h) Gill slits (viii) Osteichthyes
• 15) Prepare a list of some animals that are found parasitic on human beings.

Answers at a glance

1. 1
   Q1 → Without fundamental features, classification rests on superficial resemblance and goes wrong in three ways. First, animals that are fundamentally different would be grouped together and fundamentally alike ones separated, so a whale would be classed with fishes because both are streamlined swimmers living in water, a bat with birds because both fly, and a crocodile with fishes simply because it is a reptile that swims, when in each case the shared characters are convergent adaptations rather than evidence of common descent. Second, all the internal and decisive characters would be ignored, above all the level of organisation, the presence and type of a coelom, the type of symmetry and digestive tract and the presence of a notochord, and these internal features are far more reliable than anything visible from outside. Third, the resulting arrangement would be artificial and would indicate no natural relationships, so it could not reflect phylogeny and would carry no evolutionary information. Classification must therefore rest on fundamental features, and only then does it show the natural affinities and evolutionary relationships among animals.
2. 2
   Q2 → Non-chordates are differentiated first on the basis of the level of organisation, and then on symmetry, the germ layers and the body cavity, and these criteria are the basis of the traditional grouping used here. On the level of organisation, Porifera show a cellular level, in which the cells are arranged as independent units and are not organised into tissues, while the coelenterates and ctenophores show a tissue level, in which similar cells form tissues and perform the same function, and from the platyhelminthes onwards, as exemplified by the annelids, arthropods and molluscs, there is an organ and organ-system level, in which tissues combine into organs and organs into systems. On symmetry, some animals, such as the sponges, have no definite symmetry, and the coelenterates, ctenophores and adult echinoderms show radial symmetry, their body being divisible into equal halves along many planes, while the remaining animals, such as the annelids, arthropods and chordates, show bilateral symmetry, the body being divisible into identical left and right halves in only one plane. On the germ layers, the diploblastic animals have cells arranged in two embryonic layers, an external ectoderm and an internal endoderm, with a mesoglea in between, as in the coelenterates, and the triploblastic animals add a third layer, the mesoderm, as from the platyhelminthes onwards. On the body cavity, the acoelomates, such as the platyhelminthes, have no body cavity at all, the pseudocoelomates, such as the aschelminthes, have a cavity that is not lined by mesoderm, the mesoderm being scattered as pouches between the ectoderm and the endoderm, and the coelomates, namely the annelids, molluscs, arthropods, echinoderms, hemichordates and chordates, have a cavity that is properly lined by mesoderm. Porifera stands apart from the germ-layer scheme, since it has no true germ layers. Using these features, the animal kingdom is conventionally grouped into the invertebrate phyla, from Porifera to Hemichordata, and the phylum Chordata, which is separated from the rest because of its distinctive combination of a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail.
3. 3
   Q3 → The three terms describe the relationship between the body wall, the gut and the cavity that separates them. In a coelomate the body cavity, or coelom, is completely lined by mesoderm, so the mesoderm forms a distinct layer both outside the gut and against the body wall, and the gut is not suspended in the cavity but is enclosed by the mesodermal wall, as in Annelida. In an acoelomate, for example the platyhelminthes, the body cavity is absent altogether, so there is no space at all between the body wall and the gut. In a pseudocoelomate, for example the aschelminthes, a cavity is present but is not lined by mesoderm, because the mesoderm lies in it as scattered pouches in between the ectoderm and the endoderm, so the cavity is bounded by the endoderm on the inner side and by the ectoderm on the outer side. Hence a pseudocoelom is a body cavity that is present but incompletely lined, and it is intermediate in condition between the true coelom of a coelomate and the complete absence of a cavity in an acoelomate.
4. 4
   Q4 → The distinction lies in the site of digestion. In intracellular digestion the food particle is engulfed directly into the cells of the organism and the enzymes act inside the cell in a food vacuole, no digestive tract is formed, the process is comparatively slow because one cell must handle the whole particle, and the level of organisation concerned is tissue level; it is characteristic of sponges and is also seen in protozoans and coelenterates. In extracellular digestion the food is engulfed into a cavity or a tract lying outside the cells, digestive enzymes are secreted into that cavity, the digested food is absorbed through its wall and the undigested residue is eliminated, so the process is much faster and can handle a continuous stream of food, and the level of organisation is organ or organ-system level; it is characteristic of all animals from Cnidaria upwards, the gastrovascular cavity of a coelenterate and the alimentary canal of higher animals both serving as the site. The two are successive stages of organisation rather than mutually exclusive conditions, since in higher animals extracellular digestion first reduces the food to soluble end products, which are then absorbed into the cells and completed intracellularly.
5. 5
   Q5 → The young ones cannot be equal in number, because the two strategies carry very different costs per offspring, and those costs set the numbers. In an oviparous animal the young develop outside the mother, in eggs that are laid and then hatch in the external environment, and each egg must therefore be large, since it has to carry enough yolk to feed the embryo right up to hatching and it has to survive alone without the mother. The investment in each egg is only stored food, there is no pregnancy and no prolonged physiological burden on the mother, so a very large number of eggs can be produced. In a viviparous animal the young develop inside the mother and are nourished from her own tissues, in the mammal directly through the placenta, so each young represents a far greater investment of her tissue, energy and time, and she must also carry it to term at a high metabolic cost; she can therefore produce far fewer young. Viviparity is not, however, a single condition, since it may also mean internal development with nourishment from the yolk alone, as in the sharks, and only the placental type carries the full cost of the mother and gives the cleanest contrast. The two strategies are consequently a high-number, low-investment one and a low-number, high-investment one, and this is why a fish or a frog may shed hundreds of eggs while a mammal produces one or a very few young, each of which is far more likely to survive.
6. 6
   Q6 → The parasitic flatworms have a set of features that are all adaptations to living inside the host. The body is dorso-ventrally flattened, hence the name flatworms, and it is bilaterally symmetrical, triploblastic and acoelomate, with organ-level organisation. It is covered by a non-cellular, non-chitinous cuticle that resists the host’s enzymes, and the animal attaches to the wall of the host’s gut by hooks and suckers, which is what holds it against the peristaltic rush of the intestine so that it is not swept out. It has no digestive tract at all and simply absorbs the already digested food of the host directly through its body surface, which is why it needs no mouth and no gut. The waste is removed by specialised flame cells, which carry out both osmoregulation and excretion, since there is no excretory organ of the usual kind. The sexes are not separate, so each individual carries both male and female reproductive organs and the animal is hermaphrodite, and fertilisation is internal. The young are produced in large numbers and pass through many larval stages before reaching the adult, so the life cycle is indirect and includes an intermediate host, since the larva must first develop in one animal before it can mature in another. These features together, a body built for absorption and attachment rather than for moving and feeding, and a life cycle that needs more than one host, are what make the tapeworm and the liver fluke such effective parasites.
7. 7
   Q7 → The arthropods are the largest group because their body plan combines a set of very effective adaptations. First, the chitinous exoskeleton is a tough armour that protects the body, prevents loss of water and provides the muscle attachment points, the cost of it being that the animal must moult by ecdysis to grow and is vulnerable while the new cuticle is soft. Second, the metamerically segmented body bears jointed appendages specialised for different functions, so a single body plan serves for walking, swimming, feeding and biting, and the jointed limb gives a wide range of movement. Third, the reduced coelom is replaced functionally by the haemocoel containing haemolymph, which handles transport, and the group has solved respiration both in water and on land, with gills in the aquatic forms and tracheae, book lungs or book gills in the terrestrial ones. Fourth, the sensory equipment is excellent, with antennae and compound eyes giving mosaic vision and a statocyst for balance, so the animal detects and responds to change quickly. Fifth, the reproductive strategy is efficient, with separate sexes, internal fertilisation, high fecundity, many small eggs and often metamorphosis, so that the larva and the adult exploit different food and different habitats and do not compete, and the young disperse widely. Above all, the group occupies a great diversity of niches as herbivores, carnivores, scavengers, parasites and omnivores, in marine, freshwater and terrestrial habitats, and this adaptability is the real reason the phylum is the largest.
8. 8
   Q8 → (c) Echinodermata. The water vascular system is a unique network of fluid-filled canals in the echinoderm body, and it is used in locomotion by the action of the tube feet, in the capture and transport of food, and in respiration. Water enters through a sieve-like calcareous plate called the madreporite, passes into the stone canal, then into the ring canal around the mouth, and from the ring canal into the radial canals of each arm, and from those into the tube feet, so the system is hydraulically operated. It is absent from the larva, which is a free-swimming bilaterally symmetrical form, the Bipinnaria in the starfish and the Pluteus in the sea urchin, and appears only in the adult together with the radial symmetry. The other options are excluded because the sponges have a canal system for water and food but no water vascular system, the ctenophores have tentacles and comb plates, and no chordate possesses this system.
9. 9
   Q9 → Every vertebrate is a chordate because Vertebrata is a subphylum of the phylum Chordata, and a vertebrate therefore possesses all four chordate characters at some stage of its life, namely a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail, even where these are later modified or reduced, since the notochord is replaced by the vertebral column in the adult. But a chordate need not be a vertebrate, because the phylum Chordata is divided into three subphyla, of which only one is Vertebrata. The other two are Cephalochordata, represented by Branchiostoma, the lancelet or amphioxus, in which the notochord extends from the head to the tail region and persists throughout life, and Urochordata or Tunicata, represented by Ascidia, Salpa and Doliolum, in which the notochord is present only in the larval tail. These two groups are often called the protochordates and are exclusively marine, and since in them the notochord is a prominent, separate and persistent structure that is never replaced by a vertebral column, they are termed the invertebrate chordates. Hemichordata is not a subphylum of Chordata, being placed as a separate phylum, so it does not affect the comparison. Thus Vertebrata is wholly contained within Chordata, but Chordata extends beyond Vertebrata, which is exactly what the statement asserts.
10. 10
    Q10 → The air bladder is a gas-filled organ found in most bony fishes, and it is very important to them for several reasons. By changing the volume of gas it contains it lets the fish change its depth, since an increase in volume lowers the density and the fish rises while a decrease raises the density and it sinks, so the fish can hold any depth without swimming. It also keeps the fish buoyant and so saves the considerable energy that would otherwise be spent simply in overcoming gravity, and it acts as a resonating chamber that helps fishes such as the croaker and herring produce sound. In some fishes it is respiratory as well, for the wall is vascular and the blood gases can be exchanged across it, and in the Polypterus the air bladder functions as a lung. The qualification is that the organ is not indispensable: it is absent altogether in the cartilaginous fishes such as sharks, rays and skates, and in some bony fishes such as the mackerel it is so small that the fish must keep swimming to stay afloat. The air bladder is therefore an adaptation of the lungs of the ancestral bony fishes that became mainly hydrostatic, useful and important for locomotion and buoyancy, but not essential to life in the group as a whole.
11. 11
    Q11 → The adaptations that help a bird to fly are best listed as a set. The body is compact, streamlined and tapering at both ends, with a short neck and a small head, which reduces air resistance. The body is covered by dry, cornified, keratinised scales or feathers, the flight feathers being borne on the wings and the tail and forming the aerodynamic surface, the tail acting as the rudder, and the feathers also insulating the body and serving in courtship. The skeleton is light, with bones that are pneumatic and hollow and many of them fused, a broad sternum carrying a keel for the attachment of the large flight muscles, jaws reduced to a light horny beak with no teeth, forelimbs modified into wings, and highly mobile neck vertebrae that give the bird its wide head movement. The respiratory system is the most efficient among vertebrates, for the lungs are small and compact and are connected to nine air sacs, eight of them paired, which act as bellows so that air flows unidirectionally through the lungs during both inspiration and expiration, with no mixing of fresh and used air. The digestive system is adapted with a crop for storing and a gizzard for grinding, since there are no teeth, and the absence of a urinary bladder, with the waste passed as semisolid uric acid, keeps the body light. The reproductive system has internal fertilisation with one large yolky hard-shelled egg laid at a time, and the young hatch fully formed by direct development. Every one of these features either reduces the mass to be lifted or increases the force and the air flow available to the wing stroke.
12. 12
    Q12 → No, the numbers cannot be equal, and the reason is the difference in where the young develop and in what each mother must invest. In an oviparous mother the young develop outside the body, in eggs laid and hatched in the external environment, and the eggs are correspondingly large because each must contain enough yolk to feed the embryo to hatching and must be able to survive alone without the mother. Since the investment in each egg is only stored food, and there is no pregnancy and no prolonged burden on the mother, an oviparous mother can produce a very large number of eggs. In a viviparous mother the young develop inside the body, are attached to her and are nourished from her own tissues, either directly, as in a mammal through the placenta, or from the yolk, as in a shark. Each young therefore represents a far larger investment of the mother in tissue, energy and time, and she must carry it to term at a high metabolic cost, so she can produce far fewer young. The two strategies are consequently a high-number, low-investment and a low-number, high-investment one, and this is why a fish or a frog may lay hundreds of eggs while a mammal produces one or a few young, each of which is much more likely to survive.
13. 13
    Q13 → (c) Annelida. True metameric segmentation, in which the body is divided into a linear series of similar repeated segments or metameres, is first seen in the phylum Annelida, in which the body is divided externally into ring-like segments separated internally by septa, and each segment repeats the same structures, with its own pair of nephridia and its own share of the blood vessels and nerves. The examples are the earthworm, Nereis and the leech, though the leech shows external segmentation with the internal septa reduced or absent. Platyhelminthes and Aschelminthes, options (a) and (b), are both unsegmented, and the repeated proglottids of a tapeworm are repeated organs in an undivided body rather than true metamerism. Arthropoda, option (d), is also segmented, but its segmentation is a secondary feature of that group; the first occurrence of true segmentation in the animal kingdom is in Annelida.
14. 14
    Q14 → (a) Operculum — (viii) Osteichthyes, the bony gill cover, since the bony fishes have four pairs of gills covered by an operculum on each side. (b) Parapodia — (v) Annelida, the paired lateral paddle-like locomotory appendages of Nereis. (c) Scales — (iv) Reptilia, the dry cornified scales or scutes of the reptilian skin. (d) Comb plates — (i) Ctenophora, the ciliary plates that give the comb jellies their name and shimmer. (e) Radula — (ii) Mollusca, the rasping tongue used for feeding and drilling, as in Limax. (f) Hairs — (vii) Mammalia, the epidermal hairs of the mammalian skin, serving for insulation, sensation and display. (g) Choanocytes — (iii) Porifera, the flagellated collar cells that are the characteristic cell of the sponges. (h) Gill slits — (vi) Cyclostomata and Chondrichthyes, which bear exposed gill slits, 6-15 pairs in the jawless cyclostomes and separate and unoperculated in the cartilaginous fishes, so the operculum is left to the bony fishes.
15. 15
    Q15 → The parasites of human beings may be listed according to the organ they inhabit. In the alimentary canal and liver are the roundworms Ascaris lumbricoides, causing ascariasis, the threadworm Enterobius vermicularis, causing enterobiasis, the whipworm Trichuris trichiura, the tapeworm Taenia solium, causing taeniasis and in its larval stage cysticercosis, the liver fluke Fasciola hepatica, and the dog tapeworm Echinococcus granulosus, whose larva forms a hydatid cyst. In the blood and lymph are the malarial parasites Plasmodium vivax, P. malariae, P. ovale and P. falciparum, causing malaria, the filarial worm Wuchereria bancrofti, causing filariasis or elephantiasis, Trypanosoma gambiense and T. rhodesiense, causing African sleeping sickness, and Leishmania donovani, causing kala-azar. In the lung and the mouth region are the lung fluke Paragonimus westermani and the tongue worm Linguatula serrata, which is the parasite whose female is far longer than the male. In the skin and subcutaneous tissue are the itch mite Sarcoptes scabiei, causing scabies, and the guinea worm Dracunculus medinensis. Most of these are not specific to man, since the human shares them with other animals, and several require an intermediate host in their life cycle, as in the case of Plasmodium, of Wuchereria and of the guinea worm, the intermediate hosts being the mosquito, the mosquito and the copepod crustacean respectively.

MCQ Questions

1 Mark Questions

1. 1
   Define coelom. (Ans: body cavity lined by mesoderm)
2. 2
   Name the sensory structures of cockroaches. (Ans: antennae/pairs of compound eyes, sensory hairs)
3. 3
   Give one example of a deuterostome phylum. (Ans: Echinodermata or Chordata)
4. 4
   What is metameric segmentation? (Ans: serial repetition of body segments along the anteroposterior axis)

2 Mark Questions

1. 1
   Differentiate between radial and bilateral symmetry with one example of each. (Ans: any plane through axis gives equal halves vs only along sagittal plane)
2. 2
   Why are Platyhelminthes called flatworms? (Ans: dorsoventrally flattened, leaf-like or ribbon-like body, e.g. tapeworm/fluke)
3. 3
   Differentiate between open and closed circulatory systems with one example each. (Ans: open − blood bathes tissues (arthropods/molluscs); closed − blood flows in vessels (annelids/vertebrates))

3 Mark Questions

1. 1
   Give the salient features of phylum Annelida and mention two examples. (Ans: metamerically segmented; true coelom; closed circulation; Nereis, earthworm, leech)
2. 2
   Describe the three subphyla of Chordata with one example and the main features of each. (Ans: Urochordata/Appendicularia; Cephalochordata (Branchiostoma); Vertebrata)
3. 3
   Explain the arrangement of germ layers (dipoblastic vs triploblastic) with one example of each.

5 Mark Questions

1. 1
   State the basis of classification of the animal kingdom and elaborate on each. (Ans: levels of organisation (cellular→tissue→organ→organ system), symmetry (radial/bilateral), germ layers (diplo/triploblastic), coelom (coelomate/pseudocoelomate/acoelomate), segmentation, notochord)
2. 2
   Categorise the following into the correct phylum with two salient features each: sponge, jellyfish, earthworm, starfish, cockroach and tapeworm. (Ans: Porifera, Cnidaria, Annelida, Echinodermata, Arthropoda, Platyhelminthes)