SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Animal Kingdom

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Exercises 4.1–4.10 (part 1 of 2)

  1. Exercise 4.1

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

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    Classification would break down: with over a million species described, each animal would have to be studied and placed on its own, because only its differences of structure and form would be visible.
    Common fundamental features — arrangement of cells, body symmetry, nature of coelom, and the patterns of the digestive, circulatory and reproductive systems — are the only basis on which animals that look unlike can be grouped together.
    Without them, grouping would fall back on appearance, so animals that merely resemble each other would be put together and truly related animals separated — e.g. the sessile cylindrical polyp and the umbrella-shaped free-swimming medusa are the same coelenterates, but look nothing alike.
    Worm-shaped animals could not be told apart at all, since flatworms, roundworms and earthworms are separated by coelom (acoelomate, pseudocoelomate, coelomate), not by shape.
    A newly described species could not be assigned a systematic position, because there would be no shared characters against which to compare it.
    The whole scheme would have no levels: no phylum, class or higher grouping could be defined, so relationships between animals could not be shown or studied.

  2. Exercise 4.2

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

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    Examine the specimen for the fundamental features in this order, each one narrowing the group:
    Level of organisation — are the cells loose aggregates (cellular level, sponges), grouped into tissues (tissue level, coelenterates and ctenophores), into organs (organ level, platyhelminthes), or into functional systems (organ-system level, annelids onwards)?
    Symmetry — asymmetrical (sponges); radially symmetrical, any plane through the central axis giving two identical halves (coelenterates, ctenophores, adult echinoderms); or bilaterally symmetrical, identical left and right halves in only one plane (annelids, arthropods and higher).
    Number of germ layers — diploblastic, cells in two layers, ectoderm and endoderm with mesoglea between (coelenterates); or triploblastic, with a third layer, mesoderm (platyhelminthes to chordates).
    Coelom — coelomate (cavity lined by mesoderm), pseudocoelomate (cavity present but mesoderm only as scattered pouches), or acoelomate (no body cavity).
    Segmentation — is the body divided externally and internally into segments with serial repetition of organs (metameric segmentation, as in earthworm), or not?
    Notochord — present, so a chordate; absent, so a non-chordate (porifera to echinoderms). If a chordate, also look for the dorsal hollow nerve cord, paired pharyngeal gill slits and post-anal tail, and whether the notochord is replaced by a vertebral column (Vertebrata).
    Then fix the phylum or class from the distinctive features: digestive system complete or incomplete, circulatory system open or closed, respiratory organs, skeleton, sexes separate or not, type of fertilisation, and direct or indirect development.
    Compare the set of characters obtained with Table $\displaystyle 4.2$, which lists these salient features phylum by phylum.

  3. Exercise 4.3

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

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    Very useful — presence or absence of a cavity between the body wall and the gut wall is one of the most important characters in classification, because it splits the triploblastic animals into three clear grades.
    Coelomates — the body cavity is lined by mesoderm; this true cavity is called the coelom. Examples: annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
    Pseudocoelomates — a body cavity is present but is not lined by mesoderm; instead the mesoderm lies as scattered pouches between ectoderm and endoderm. Example: aschelminthes.
    Acoelomates — the body cavity is altogether absent. Example: platyhelminthes.
    Its value is that it is a fixed embryonic character, not an external one, so it separates animals that outwardly look the same: flatworms, roundworms and earthworms are all worm-shaped, yet are acoelomate, pseudocoelomate and coelomate respectively.
    It therefore places an unknown triploblastic specimen in the right broad group before any other character is considered.

  4. Exercise 4.4

    Distinguish between intracellular and extracellular digestion?

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    The difference is where the food is broken down — inside the cell, or in a cavity outside the cells.
    Intracellular digestion
    Food is taken into the cell and digested within the cell itself.
    No digestive cavity is needed, so it is found in animals with the simplest organisation.
    Example: in Porifera (sponges) digestion is wholly intracellular; food is gathered by the water transport (canal) system and the choanocytes lining the spongocoel and canals take it in.
    Extracellular digestion
    Food is digested outside the cells, in a digestive cavity or in the lumen of the alimentary canal, and the digested products are then absorbed by the cells.
    It allows larger food to be handled, so animals with a gut show it.
    Example: coelenterates digest food in the central gastro-vascular cavity, which has a single opening, the mouth on the hypostome.
    Coelenterates and ctenophores show both — digestion in them is extracellular and intracellular; extracellular alone is the rule in animals with a complete alimentary canal (aschelminthes onwards).

  5. Exercise 4.5

    What is the difference between direct and indirect development?

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    The difference is whether a larva comes in between the egg and the adult.
    Direct development
    The young one that hatches or is born already resembles the adult; it simply grows in size.
    There is no larval stage and no metamorphosis.
    Examples: Osteichthyes, Reptilia, Aves and Mammalia; aschelminthes may also develop directly.
    Indirect development
    Development passes through one or more larval stages that are morphologically distinct from the adult, the larva changing into the adult form by metamorphosis.
    Examples: Porifera, Ctenophora, Mollusca, Echinodermata (free-swimming larva), Hemichordata and Amphibia; platyhelminthes develop through many larval stages.
    In some groups either is possible — in arthropods and in aschelminthes development may be direct or indirect.

  6. Exercise 4.6

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

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    The parasitic flatworms are built for attachment to a host and for absorbing food from it.
    Hooks and suckers are present in the parasitic forms, for clinging to the host's tissue.
    They are mostly endoparasites, found in animals including human beings.
    Some of them absorb nutrients from the host directly through their body surface, so no gut of their own is needed.
    The body is dorso-ventrally flattened (hence flatworms), which gives a large surface area in contact with the host's digested food.
    Specialised flame cells carry out osmoregulation and excretion.
    Sexes are not separate (hermaphrodite) and fertilisation is internal — a single worm inside a host can still reproduce.
    Development is through many larval stages, which helps in passing from one host to the next.
    Examples: Taenia (tapeworm) and Fasciola (liver fluke).

  7. Exercise 4.7

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

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    Arthropoda is the largest phylum of Animalia — over two-thirds of all named species on earth are arthropods — because its body plan fits almost every habitat and way of life:
    A chitinous exoskeleton covers the body: it protects, supports, and checks water loss, which is what makes life on dry land possible.
    Jointed appendages (arthros — joint, poda — appendages) can be modified for walking, swimming, flying, feeding or sensing, so one basic limb serves many purposes.
    The body is divided into head, thorax and abdomen, giving a division of labour between the regions.
    Respiratory organs vary — gills, book gills, book lungs or a tracheal system — so different arthropods can breathe in water, on land or in air.
    Well-developed sensory organs — antennae, compound and simple eyes, and statocysts (balancing organs) — let them find food and avoid danger.
    Excretion through malpighian tubules conserves water, again suiting terrestrial life.
    They are mostly dioecious with internal fertilisation and are mostly oviparous, so large numbers of offspring are produced.
    Development may be direct or indirect; where it is indirect, larva and adult live and feed differently, so they do not compete with each other.
    The range of the phylum shows the result: honey bee, silkworm, lac insect, mosquitoes, locust and the king crab (a living fossil) are all arthropods.

  8. Exercise 4.8

    Water vascular system is the characteristic of which group of the following:
    (a)
    Porifera
    (b)
    Ctenophora
    (c)
    Echinodermata
    (d)
    Chordata

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    (c) Echinodermata.
    The water vascular system is the most distinctive feature of echinoderms.
    It helps in locomotion, capture and transport of food, and respiration.
    Examples: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).
    The other three are wrong: Porifera has a water transport (canal) system with ostia, spongocoel and osculum — not a water vascular system; Ctenophora has comb plates for locomotion; Chordata is defined by notochord, dorsal hollow nerve cord and gill slits.

  9. Exercise 4.9

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

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    The statement is true because Vertebrata is only one of the three subphyla of the phylum Chordata.
    All vertebrates are chordates: members of subphylum Vertebrata possess a notochord during the embryonic period, along with the other chordate characters — dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail — so they satisfy the definition of a chordate.
    In vertebrates the notochord is later replaced by a cartilaginous or bony vertebral column in the adult, and they have in addition a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages which may be fins or limbs.
    All chordates are not vertebrates: the other two subphyla, Urochordata (Tunicata) and Cephalochordata, together called protochordates, never form a vertebral column.
    In Urochordata the notochord is present only in the larval tail (e.g. Ascidia, Salpa, Doliolum); in Cephalochordata it extends from head to tail and persists throughout life (e.g. Branchiostoma, the lancelet).
    These animals have a notochord and so are chordates, but having no vertebral column they are not vertebrates — hence all vertebrates are chordates, but all chordates are not vertebrates.

  10. Exercise 4.10

    How important is the presence of air bladder in Pisces?

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    Very important — the air bladder regulates buoyancy, letting the fish stay at a chosen depth without effort.
    Osteichthyes (bony fishes) possess an air bladder, so they can float at a level in the water and need not swim merely to keep from sinking; energy is saved for feeding and escape.
    Chondrichthyes (cartilaginous fishes) have no air bladder, and because of its absence they have to swim constantly to avoid sinking.
    The contrast between the two classes of Pisces is itself the proof of how useful the structure is.