class 11 notes

Class 11 Biology Notes Chapter 4 (Animal kingdom) – Biology Book

Philoid

Philoid

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Biology
Alright class, let's dive straight into Chapter 4: Animal Kingdom. This chapter is fundamental because it lays the groundwork for understanding the incredible diversity of animal life on Earth and how scientists systematically categorize it. For your government exam preparation, focus on the basis of classification and the key distinguishing features of each phylum.

Animal Kingdom (Chapter 4) - Detailed Notes

1. Basis of Classification

Animals are classified based on fundamental features like:

  • (a) Levels of Organisation:

    • Cellular Level: Cells are arranged as loose aggregates. Basic division of labour among cells. (e.g., Phylum Porifera - Sponges)
    • Tissue Level: Cells performing the same function are arranged into tissues. (e.g., Phylum Coelenterata/Cnidaria, Phylum Ctenophora)
    • Organ Level: Tissues are grouped together to form organs, each specialized for a particular function. (e.g., Phylum Platyhelminthes)
    • Organ System Level: Organs associate to form functional systems, each concerned with a specific physiological function. This is seen in higher animals. (e.g., Annelids, Arthropods, Molluscs, Echinoderms, Chordates)
      • Note: Organ systems can show varying complexities (e.g., Digestive System: Incomplete - single opening vs. Complete - two openings; Circulatory System: Open vs. Closed).

  • (b) Symmetry:

    • Asymmetrical: Any plane passing through the centre does not divide the body into equal halves. (e.g., Most Sponges)
    • Radial Symmetry: Any plane passing through the central axis divides the organism into two identical halves. Suitable for sessile or slow-moving lifestyle. (e.g., Coelenterates, Ctenophores, Adult Echinoderms)
    • Bilateral Symmetry: The body can be divided into identical left and right halves by only one plane. Associated with cephalization (formation of a head). (e.g., Platyhelminthes to Chordates)

  • (c) Diploblastic and Triploblastic Organisation: Refers to the number of embryonic germ layers.

    • Diploblastic: Cells arranged in two embryonic layers – external Ectoderm and internal Endoderm. An undifferentiated layer, Mesoglea, may be present in between. (e.g., Coelenterates, Ctenophores)
    • Triploblastic: Three embryonic layers – Ectoderm, Mesoderm (middle layer), and Endoderm. Mesoderm gives rise to most organs. (e.g., Platyhelminthes to Chordates)

  • (d) Coelom (Body Cavity): The nature of the cavity between the body wall and the gut wall is important for classification.

    • Acoelomates: Animals in which the body cavity is absent. The space between ectoderm and endoderm is filled with parenchyma (mesoderm-derived tissue). (e.g., Platyhelminthes)
    • Pseudocoelomates: The body cavity is not lined by mesoderm. Instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm. (e.g., Aschelminthes/Nematoda)
    • Coelomates: Possess a true coelom – a cavity lined by mesoderm on all sides. Allows organs to be better organized and supported. (e.g., Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata)

  • (e) Segmentation (Metamerism):

    • The body is externally and internally divided into segments (metameres) with a serial repetition of at least some organs.
    • True metamerism is seen in Phylum Annelida (e.g., Earthworm), Arthropoda, and Chordata.

  • (f) Notochord:

    • A mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
    • Animals with notochord are called Chordates.
    • Animals without notochord are called Non-chordates (Porifera to Echinodermata/Hemichordata).

2. Classification of Animals (Phylum-wise Key Features & Examples)

(Focus on distinguishing features for exam purposes)

  • Phylum Porifera (Sponges):

    • Cellular level of organisation.
    • Asymmetrical (mostly).
    • Water Canal System (or Water Transport System): Unique feature. Water enters through minute pores (Ostia) into a central cavity (Spongocoel) and goes out through the Osculum. Helps in food gathering, respiration, and waste removal.
    • Choanocytes (Collar cells) line the spongocoel and canals.
    • Skeleton of spicules or spongin fibres.
    • Hermaphrodite; Internal fertilization; Indirect development (larval stage).
    • Examples: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge).

  • Phylum Coelenterata (Cnidaria):

    • Tissue level of organisation; Diploblastic; Radial symmetry.
    • Cnidoblasts or Cnidocytes: Unique stinging cells containing nematocysts, present on tentacles and body. Used for anchorage, defense, and capture of prey.
    • Gastro-vascular cavity (coelenteron) with a single opening (Hypostome) serving as mouth and anus.
    • Polyp (sessile, cylindrical, e.g., Hydra, Adamsia) and Medusa (free-swimming, umbrella-shaped, e.g., Aurelia - jellyfish) forms exist.
    • Alternation of generation (Metagenesis) seen in some (e.g., Obelia - Polyp produces medusae asexually, Medusa forms polyps sexually).
    • Corals have a skeleton composed of calcium carbonate (e.g., Meandrina - Brain coral).
    • Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina.

  • Phylum Ctenophora (Comb Jellies / Sea Walnuts):

    • Tissue level; Diploblastic; Radial symmetry.
    • Eight external rows of ciliated Comb Plates: Help in locomotion.
    • Bioluminescence: Characteristic feature (ability to emit light).
    • Lack cnidoblasts (unlike Cnidaria), but have colloblasts (sticky cells) for prey capture.
    • Only sexual reproduction; External fertilization; Indirect development.
    • Examples: Pleurobrachia, Ctenoplana.

  • Phylum Platyhelminthes (Flatworms):

    • Organ level; Triploblastic; Bilateral symmetry; Acoelomate.
    • Dorso-ventrally flattened body.
    • Mostly endoparasites; some free-living.
    • Hooks and suckers present in parasitic forms.
    • Flame cells (Protonephridia): Specialized cells for osmoregulation and excretion.
    • Incomplete digestive system (absent in tapeworms).
    • Hermaphrodite; Internal fertilization; Indirect development (many larval stages).
    • Examples: Taenia (Tapeworm), Fasciola (Liver fluke), Planaria (shows high regeneration capacity).

  • Phylum Aschelminthes (Roundworms):

    • Organ system level; Triploblastic; Bilateral symmetry; Pseudocoelomate.
    • Body circular in cross-section; unsegmented.
    • Complete digestive system (mouth and anus).
    • Excretory tube removes waste through an excretory pore.
    • Sexes are separate (Dioecious); often show sexual dimorphism (females longer than males).
    • Internal fertilization; Development may be direct or indirect.
    • Many are parasitic, some free-living.
    • Examples: Ascaris (Roundworm), Wuchereria (Filarial worm - causes elephantiasis), Ancylostoma (Hookworm).

  • Phylum Annelida (Segmented Worms):

    • Organ system level; Triploblastic; Bilateral symmetry; Coelomate.
    • Metameric Segmentation: Body surface distinctly marked out into segments or metameres.
    • Longitudinal and circular muscles help in locomotion.
    • Aquatic annelids (like Nereis) have lateral appendages, Parapodia, for swimming.
    • Closed circulatory system (first phylum to exhibit this).
    • Nephridia: Organs for osmoregulation and excretion.
    • Neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
    • Nereis is dioecious; Earthworms and Leeches are monoecious (hermaphrodite).
    • Examples: Nereis, Pheretima (Earthworm), Hirudinaria (Blood-sucking Leech).

  • Phylum Arthropoda (Jointed-legged Animals):

    • Largest phylum of Animalia.
    • Organ system level; Triploblastic; Bilateral symmetry; Coelomate; Segmented.
    • Exoskeleton made of Chitin.
    • Body divisible into Head, Thorax, and Abdomen.
    • Jointed appendages (arthros=joint, poda=appendages) – used for various functions.
    • Respiratory organs: Gills (aquatic), Book gills (e.g., Limulus), Book lungs (e.g., Scorpion, Spider), or Tracheal system (insects).
    • Open circulatory system.
    • Sensory organs like antennae, eyes (compound and simple), statocysts (balance organs) are present.
    • Excretion through Malpighian tubules (insects) or green glands/antennal glands (crustaceans).
    • Mostly dioecious; Internal fertilization; Mostly oviparous; Development direct or indirect.
    • Economic Importance: Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect); Vectors: Anopheles, Culex, Aedes (Mosquitoes); Gregarious Pest: Locusta (Locust); Living fossil: Limulus (King crab).
    • Examples: Spiders, Scorpions, Crabs, Prawns, Insects (Cockroach, Butterfly, Mosquito).

  • Phylum Mollusca (Soft-bodied Animals):

    • Second largest phylum.
    • Organ system level; Triploblastic; Bilateral symmetry (some asymmetrical due to torsion); Coelomate.
    • Body is unsegmented, covered by a calcareous shell (if present).
    • Body divisible into Head, Visceral Hump, and Muscular Foot.
    • Mantle: A soft, spongy layer of skin covering the visceral hump. The space between the mantle and hump is the Mantle Cavity, where feather-like gills (ctenidia) are present (respiratory and excretory functions).
    • Radula: File-like rasping organ in the mouth for feeding (absent in bivalves).
    • Open circulatory system (except Cephalopods like squid, octopus).
    • Dioecious; Oviparous; Indirect development.
    • Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea hare), Dentalium (Tusk shell), Chaetopleura (Chiton).

  • Phylum Echinodermata (Spiny-skinned Animals):

    • Exclusively marine.
    • Organ system level; Triploblastic; Coelomate.
    • Adults show Radial symmetry (pentamerous), but larvae are Bilaterally symmetrical.
    • Endoskeleton of calcareous ossicles (spiny skinned).
    • Water Vascular System: Distinctive feature. Helps in locomotion, capture and transport of food, and respiration. Consists of tube feet (podia).
    • Digestive system complete; Mouth on ventral side, Anus on dorsal side.
    • No specialized excretory system.
    • Sexes separate; External fertilization; Indirect development (free-swimming larva).
    • Remarkable power of regeneration.
    • Examples: Asterias (Starfish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), Ophiura (Brittle star).

  • Phylum Hemichordata:

    • Earlier considered a sub-phylum under Chordata, now placed as a separate phylum under Non-chordata.
    • Worm-like marine animals.
    • Organ system level; Triploblastic; Bilateral symmetry; Coelomate.
    • Body cylindrical, divisible into Proboscis, Collar, and Trunk.
    • Open circulatory system.
    • Respiration through gills.
    • Excretory organ is Proboscis gland.
    • Sexes separate; External fertilization; Indirect development.
    • Possess a rudimentary structure in the collar region called Stomochord (similar to notochord, but not homologous).
    • Examples: Balanoglossus, Saccoglossus.

  • Phylum Chordata:

    • Characterized by the presence of:

      1. Notochord (at least during early embryonic life).
      2. Dorsal, Hollow Nerve Cord.
      3. Paired Pharyngeal Gill Slits.
      4. Post-anal Tail.

    • Bilateral symmetry; Triploblastic; Coelomate; Organ system level.

    • Closed circulatory system.

    • Differences between Chordates and Non-chordates:

      Feature Chordates Non-chordates
      Notochord Present (at some stage) Absent
      Central Nervous Sys Dorsal, Hollow, Single Ventral, Solid, Double
      Pharyngeal Gill Slits Present (at some stage) Absent
      Heart Ventral (if present) Dorsal (if present)
      Post-anal Tail Present (at some stage) Absent

    • Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata.

    • Subphylum Urochordata (Tunicata):

      • Notochord present only in larval tail.
      • Body covered by 'tunic'. Exclusively marine.
      • Examples: Ascidia, Salpa, Doliolum.

    • Subphylum Cephalochordata:

      • Notochord extends from head to tail region and persists throughout life.
      • Fish-like marine animals.
      • Examples: Branchiostoma (Amphioxus or Lancelet).

    • Subphylum Vertebrata:

      • Possess notochord during the embryonic period; replaced by a cartilaginous or bony Vertebral Column in the adult.

      • "All vertebrates are chordates, but all chordates are not vertebrates."

      • Ventral muscular heart (2, 3, or 4 chambers).

      • Kidneys for excretion & osmoregulation.

      • Paired appendages (fins or limbs).

      • Vertebrata Divisions & Classes:

        • Division Agnatha (Jawless):

          • Class Cyclostomata:
            • Ectoparasites on some fishes. Elongated body.
            • Sucking and circular mouth without jaws.
            • 6-15 pairs of gill slits for respiration. No scales; cartilaginous cranium and vertebral column.
            • Closed circulation. Marine, but migrate to freshwater for spawning (anadromous). Larva (ammocoete) undergoes metamorphosis and returns to ocean.
            • Examples: Petromyzon (Lamprey), Myxine (Hagfish).

        • Division Gnathostomata (Bear Jaws):

          • Superclass Pisces (Bear Fins):

            • Class Chondrichthyes (Cartilaginous Fishes):
              • Marine; Streamlined body; Cartilaginous endoskeleton.
              • Mouth ventral. Gill slits separate, without operculum (gill cover) (except Chimaeras).
              • Skin tough, with minute placoid scales.
              • Predaceous. Lack air bladder (swim constantly).
              • Heart 2-chambered (1 auricle, 1 ventricle). Poikilothermous (cold-blooded).
              • Sexes separate. Internal fertilization. Many viviparous.
              • Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great white shark), Trygon (Stingray - has poison sting), Torpedo (Electric ray - has electric organs).
            • Class Osteichthyes (Bony Fishes):
              • Marine and freshwater; Streamlined body; Bony endoskeleton.
              • Mouth mostly terminal. Four pairs of gills covered by an operculum.
              • Skin covered with cycloid/ctenoid scales.
              • Air bladder present (regulates buoyancy).
              • Heart 2-chambered. Poikilothermous (cold-blooded).
              • Sexes separate. External fertilization. Mostly oviparous. Direct development.
              • Examples: Marine - Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater - Labeo (Rohu), Catla (Katla), Clarias (Magur); Aquarium - Betta (Fighting fish), Pterophyllum (Angel fish).

          • Superclass Tetrapoda (Bear Limbs):

            • Class Amphibia:
              • Live in aquatic and terrestrial habitats (need water for reproduction).
              • Body divisible into Head and Trunk (tail may be present in some). Skin moist, without scales.
              • Two pairs of limbs (if present).
              • Respiration by gills (larva), lungs, and skin (adult).
              • Heart 3-chambered (2 auricles, 1 ventricle). Poikilothermous.
              • Tympanum represents the ear. Eyes have eyelids.
              • Cloaca: Common chamber for alimentary, urinary, and reproductive tracts.
              • Sexes separate. External fertilization. Oviparous. Indirect development (tadpole larva).
              • Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).
            • Class Reptilia:
              • Mostly terrestrial. Body covered by dry, cornified skin with epidermal scales or scutes.
              • Creeping or crawling mode of locomotion.
              • Limbs, when present, are two pairs.
              • Tympanum represents ear.
              • Heart usually 3-chambered, but 4-chambered in Crocodiles. Poikilothermous.
              • Respiration by lungs.
              • Internal fertilization. Oviparous. Direct development. Cleidoic eggs (shelled, amniotic eggs) - adaptation for land.
              • Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator, Hemidactylus (Wall lizard). Poisonous snakes: Naja (Cobra), Bangarus (Krait), Vipera (Viper). Non-poisonous: Python.
            • Class Aves (Birds):
              • Presence of feathers; most can fly (except flightless birds like Ostrich, Kiwi, Penguin). Possess beak.
              • Forelimbs modified into wings. Hind limbs generally have scales, modified for walking, swimming, or perching.
              • Endoskeleton bony, long bones hollow with air cavities (pneumatic bones) - adaptation for flight.
              • Skin dry, without glands except oil gland at the base of the tail.
              • Digestive tract has additional chambers: Crop and Gizzard.
              • Heart completely 4-chambered. Homeothermous (warm-blooded).
              • Respiration by lungs. Air sacs connected to lungs supplement respiration.
              • Internal fertilization. Oviparous. Direct development. Well-developed parental care.
              • Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).
            • Class Mammalia:
              • Found in diverse habitats.
              • Presence of Mammary Glands (milk-producing) - unique feature.
              • Two pairs of limbs adapted for various functions.
              • Skin possesses Hair. External ears (pinnae) present.
              • Different types of teeth in the jaw (heterodont, thecodont, diphyodont).
              • Heart 4-chambered. Homeothermous.
              • Respiration by lungs.
              • Sexes separate. Internal fertilization. Viviparous (mostly, exceptions: platypus, echidna are oviparous). Direct development. Extensive parental care.
              • Examples: Oviparous - Ornithorhynchus (Platypus), Tachyglossus (Echidna); Viviparous - Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), Panthera leo (Lion), Homo sapiens (Human).

Multiple Choice Questions (MCQs)

  1. Which of the following is a characteristic feature exclusive to the Phylum Porifera?
    (A) Presence of tissues
    (B) Water Canal System
    (C) Bilateral symmetry
    (D) Presence of cnidoblasts

  2. Identify the pseudocoelomate animal from the following:
    (A) Taenia (Tapeworm)
    (B) Pheretima (Earthworm)
    (C) Ascaris (Roundworm)
    (D) Fasciola (Liver fluke)

  3. Metameric segmentation is a characteristic feature first observed in which phylum?
    (A) Platyhelminthes
    (B) Arthropoda
    (C) Annelida
    (D) Mollusca

  4. The file-like rasping organ for feeding in molluscs is called:
    (A) Mantle
    (B) Ctenidia
    (C) Radula
    (D) Operculum

  5. Adult Echinoderms exhibit _______ symmetry, while their larvae exhibit _______ symmetry.
    (A) Radial, Bilateral
    (B) Bilateral, Radial
    (C) Radial, Radial
    (D) Bilateral, Bilateral

  6. Which of the following is NOT a fundamental character of Chordates?
    (A) Dorsal hollow nerve cord
    (B) Ventral solid nerve cord
    (C) Pharyngeal gill slits
    (D) Notochord

  7. Which class of Vertebrata is characterized by a cartilaginous endoskeleton, placoid scales, and absence of an air bladder?
    (A) Cyclostomata
    (B) Osteichthyes
    (C) Chondrichthyes
    (D) Amphibia

  8. Pneumatic bones and feathers are characteristic features of:
    (A) Reptilia
    (B) Aves
    (C) Mammalia
    (D) Amphibia

  9. Which of the following mammals is oviparous (lays eggs)?
    (A) Macropus (Kangaroo)
    (B) Balaenoptera (Blue Whale)
    (C) Pteropus (Flying Fox)
    (D) Ornithorhynchus (Platypus)

  10. Malpighian tubules are the excretory organs in:
    (A) Annelida
    (B) Mollusca
    (C) Arthropoda (Insects)
    (D) Echinodermata

Answer Key:

  1. (B)
  2. (C)
  3. (C)
  4. (C)
  5. (A)
  6. (B)
  7. (C)
  8. (B)
  9. (D)
  10. (C)

Make sure you revise these key features repeatedly. Understanding the basis of classification and the unique characteristics of each phylum is crucial for tackling questions from this chapter. Good luck with your preparation!

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