Animal Kingdom Notes

Animal Kingdom

When observing the animal kingdom, the diversity in structure and form necessitates classification.

  • Over a million animal species have been described.
  • Classification is essential for assigning systematic positions to newly discovered species.

4.1 Basis of Classification

Despite structural differences, fundamental features are common among animals, including:

  • Cell arrangement
  • Body symmetry
  • Coelom nature
  • Digestive, circulatory, and reproductive systems

These features form the basis for animal classification.

4.1.1 Levels of Organisation

Animalia members are multicellular but exhibit varying cellular organisation:

  • Cellular Level:
    • Seen in sponges.
    • Cells are arranged as loose aggregates.
    • Division of labor occurs among cells.
  • Tissue Level:
    • Seen in coelenterates.
    • Cells performing similar functions are arranged into tissues.
  • Organ Level:
    • Seen in Platyhelminthes and higher phyla.
    • Tissues form organs specialized for specific functions.
  • Organ System Level:
    • Seen in annelids, arthropods, molluscs, echinoderms, and chordates.
    • Organs associate to form functional systems, each for a specific physiological function.
    • Organ systems show varying complexities.
      • Incomplete Digestive System:
        • Platyhelminthes possess this system.
        • It has a single opening for both mouth and anus.
      • Complete Digestive System:
        • Has two openings: mouth and anus.
      • Circulatory System:
        • Open Type: Blood is pumped out of the heart, and cells/tissues are directly bathed in it.
        • Closed Type: Blood circulates through vessels of varying diameters (arteries, veins, capillaries).

4.1.2 Symmetry

Animals are categorized based on symmetry:

  • Asymmetry:
    • Sponges are mostly asymmetrical.
    • No plane through the center divides them into equal halves.
  • Radial Symmetry:
    • Any plane passing through the central axis divides the organism into two identical halves.
    • Coelenterates, ctenophores, and echinoderms exhibit this symmetry.
  • Bilateral Symmetry:
    • The body can be divided into identical left and right halves in only one plane.
    • Annelids and arthropods have bilateral symmetry.

4.1.3 Diploblastic and Triploblastic Organisation

  • Diploblastic Animals:
    • Cells arranged in two embryonic layers: ectoderm (external) and endoderm (internal).
    • Coelenterates are diploblastic.
    • Mesoglea (undifferentiated layer) is present between the ectoderm and endoderm.
  • Triploblastic Animals:
    • Have three germinal layers: ectoderm, mesoderm, and endoderm.
    • Platyhelminthes to chordates are triploblastic.

4.1.4 Coelom

  • Presence or absence of a cavity between the body wall and gut wall is crucial in classification.
  • Coelom: Body cavity lined by mesoderm.
  • Coelomates: Animals possessing a coelom (e.g., annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates).
  • Pseudocoelom: Body cavity not lined by mesoderm; mesoderm present as scattered pouches between ectoderm and endoderm.
  • Pseudocoelomates: Animals possessing a pseudocoelom (e.g., aschelminthes).
  • Acoelomates: Animals lacking a body cavity (e.g., platyhelminthes).

4.1.5 Segmentation

  • In some animals, the body is divided into segments externally and internally, with serial repetition of organs.
  • Metameric Segmentation: Seen in earthworms, where the body shows this pattern.
  • Metamerism: The phenomenon of metameric segmentation.

4.1.6 Notochord

  • Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
  • Chordates: Animals with a notochord.
  • Non-chordates: Animals lacking a notochord (e.g., porifera to echinoderms).

4.2 Classification of Animals

Broad classification of Animalia is based on fundamental features:

4.2.1 Phylum - Porifera

  • Commonly known as sponges.
  • Generally marine and mostly asymmetrical animals.
  • Primitive multicellular animals with cellular level organisation.
  • Water Transport/Canal System: Water enters through ostia (minute pores) into a central cavity (spongocoel) and exits through the osculum.
    • This system aids food gathering, respiratory exchange, and waste removal.
  • Choanocytes (Collar Cells): Line the spongocoel and canals.
  • Digestion is intracellular.
  • The body is supported by spicules or spongin fibres.
  • Sexes are not separate (hermaphrodite); eggs and sperms are produced by the same individual.
  • Reproduction:
    • Asexual: Fragmentation.
    • Sexual: Formation of gametes.
  • Fertilisation is internal, and development is indirect, including a morphologically distinct larval stage.
  • Examples: Sycon (Scypha), Spongilla (freshwater sponge), and Euspongia (bath sponge).

4.2.2 Phylum – Coelenterata (Cnidaria)

  • Aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals.
  • Derived name from cnidoblasts or cnidocytes containing stinging capsules (nematocysts) on tentacles and body.
    • Cnidoblasts are used for anchorage, defense, and prey capture.
  • Tissue level organisation and are diploblastic.
  • Central gastro-vascular cavity with a single opening (mouth) on hypostome.
  • Digestion is extracellular and intracellular.
  • Some cnidarians (e.g., corals) have a skeleton composed of calcium carbonate.
  • Body Forms: Polyp and Medusa.
    • Polyp: Sessile and cylindrical form (e.g., Hydra, Adamsia).
    • Medusa: Umbrella-shaped and free-swimming (e.g., Aurelia or jellyfish).
  • Metagenesis (Alternation of Generations): Some cnidarians exhibit this, where polyps produce medusae asexually, and medusae form polyps sexually (e.g., Obelia).
  • Examples: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan), and Meandrina (brain coral).

4.2.3 Phylum – Ctenophora

  • Commonly known as sea walnuts or comb jellies.
  • Exclusively marine, radially symmetrical, diploblastic organisms with tissue level organisation.
  • Body bears eight external rows of ciliated comb plates for locomotion.
  • Digestion is both extracellular and intracellular.
  • Bioluminescence (property of emitting light) is well-marked.
  • Sexes are not separate.
  • Reproduction takes place only by sexual means.
  • Fertilisation is external with indirect development.
  • Examples: Pleurobrachia and Ctenoplana.

4.2.4 Phylum – Platyhelminthes

  • Dorso-ventrally flattened body, hence called flatworms.
  • Mostly endoparasites found in animals, including human beings.
  • Bilaterally symmetrical, triploblastic, and acoelomate animals with organ level organisation.
  • Hooks and suckers are present in parasitic forms.
  • Some absorb nutrients directly from the host through their body surface.
  • Specialized cells called flame cells help in osmoregulation and excretion.
  • Sexes are not separate.
  • Fertilisation is internal, and development is through many larval stages.
  • Some members like Planaria possess high regeneration capacity.
  • Examples: Taenia (tapeworm) and Fasciola (liver fluke).

4.2.5 Phylum – Aschelminthes

  • Body is circular in cross-section, hence the name roundworms.
  • May be free-living, aquatic, terrestrial, or parasitic in plants and animals.
  • Organ-system level of body organisation.
  • Bilaterally symmetrical, triploblastic, and pseudocoelomate animals.
  • Alimentary canal is complete with a well-developed muscular pharynx.
  • An excretory tube removes body wastes from the body cavity through the excretory pore.
  • Sexes are separate (dioecious), i.e., males and females are distinct.
  • Often females are longer than males.
  • Fertilisation is internal, and development may be direct (young ones resemble the adult) or indirect.
  • Examples: Ascaris (roundworm), Wuchereria (filaria worm), and Ancylostoma (hookworm).

4.2.6 Phylum – Annelida

  • May be aquatic (marine and freshwater) or terrestrial; free-living, and sometimes parasitic.
  • Exhibit organ-system level of body organisation and bilateral symmetry.
  • Triploblastic, metamerically segmented, and coelomate animals.
  • Body surface is distinctly marked out into segments or metameres; hence, the phylum name Annelida (Latin, annulus: little ring).
  • Possess longitudinal and circular muscles for locomotion.
  • Aquatic annelids like Nereis possess lateral appendages, parapodia, for swimming.
  • A closed circulatory system is present.
  • Nephridia help in osmoregulation and excretion.
  • The neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
  • Nereis is dioecious, but earthworms and leeches are monoecious.
  • Reproduction is sexual.
  • Examples: Nereis, Pheretima (earthworm), and Hirudinaria (blood-sucking leech).

4.2.7 Phylum – Arthropoda

  • Largest phylum of Animalia, including insects.
  • Over two-thirds of all named species on earth are arthropods.
  • Organ-system level of organisation.
  • Bilaterally symmetrical, triploblastic, segmented, and coelomate animals.
  • The body is covered by a chitinous exoskeleton.
  • The body consists of head, thorax, and abdomen.
  • Have jointed appendages (arthros-joint, poda-appendages).
  • Respiratory organs are gills, book gills, book lungs, or tracheal system.
  • The circulatory system is of open type.
  • Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present.
  • Excretion takes place through malpighian tubules.
  • Mostly dioecious.
  • Fertilisation is usually internal.
  • Mostly oviparous.
  • Development may be direct or indirect.
  • Examples:
    • Economically important insects: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect).
    • Vectors: Anopheles, Culex, and Aedes (mosquitoes).
    • Gregarious pest: Locusta (locust).
    • Living fossil: Limulus (king crab).

4.2.8 Phylum – Mollusca

  • Second largest animal phylum.
  • Terrestrial or aquatic (marine or freshwater) having an organ-system level of organisation.
  • Bilaterally symmetrical, triploblastic, and coelomate animals.
  • The body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot, and visceral hump.
  • The mantle, a soft and spongy layer of skin, forms over the visceral hump.
  • The space between the hump and the mantle is called the mantle cavity, where feather-like gills are present for respiratory and excretory functions.
  • The anterior head region has sensory tentacles.
  • The mouth contains a file-like rasping organ for feeding, called the radula.
  • Usually dioecious and oviparous with indirect development.
  • Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea hare), Dentalium (tusk shell), and Chaetopleura (chiton).

4.2.9 Phylum – Echinodermata

  • Animals have an endoskeleton of calcareous ossicles, hence the name Echinodermata (spiny-bodied).
  • All are marine with organ-system level of organisation.
  • Adults are radially symmetrical, but larvae are bilaterally symmetrical.
  • Triploblastic and coelomate animals.
  • The digestive system is complete with mouth on the lower (ventral) side and anus on the upper (dorsal) side.
  • The most distinctive feature is the presence of a water vascular system, which helps in locomotion, capture and transport of food, and respiration.
  • An excretory system is absent.
  • Sexes are separate.
  • Reproduction is sexual.
  • Fertilisation is usually external.
  • Development is indirect with free-swimming larva.
  • Examples: Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), and Ophiura (brittle star).

4.2.10 Phylum – Hemichordata

  • Was earlier considered as a sub-phylum under phylum Chordata but is now placed as a separate phylum under non-chordata.
  • Hemichordates have a rudimentary structure in the collar region called the stomochord, a structure similar to the notochord.
  • Small group of worm-like marine animals with organ-system level of organisation.
  • Bilaterally symmetrical, triploblastic, and coelomate animals.
  • The body is cylindrical and is composed of an anterior proboscis, a collar, and a long trunk.
  • The circulatory system is of open type.
  • Respiration takes place through gills.
  • The excretory organ is the proboscis gland.
  • Sexes are separate.
  • Fertilisation is external.
  • Development is indirect.
  • Examples: Balanoglossus and Saccoglossus.

4.2.11 Phylum – Chordata

  • Fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits.
  • Bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation.
  • Possess a post-anal tail and a closed circulatory system.
  • Divided into three subphyla: Urochordata or Tunicata, Cephalochordata, and Vertebrata.
  • Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine.
  • In Urochordata, the notochord is present only in the larval tail.
  • In Cephalochordata, it extends from head to tail region and is persistent throughout their life.
  • Examples:
    • Urochordata – Ascidia, Salpa, Doliolum.
    • Cephalochordata – Branchiostoma (Amphioxus or Lancelet).
Vertebrata
  • Possess notochord during the embryonic period; it is replaced by a cartilaginous or bony vertebral column in the adult.
  • All vertebrates are chordates, but not all chordates are vertebrates.
  • Have a ventral muscular heart with two, three, or four chambers, kidneys for excretion and osmoregulation, and paired appendages (fins or limbs).
4.2.11.1 Class – Cyclostomata
  • All living members are ectoparasites on some fishes.
  • Have an elongated body bearing 6-15 pairs of gill slits for respiration.
  • Have a sucking and circular mouth without jaws.
  • The body is devoid of scales and paired fins.
  • Cranium and vertebral column are cartilaginous.
  • Circulation is of closed type.
  • Marine but migrate for spawning to freshwater; die after spawning.
  • Larvae return to the ocean after metamorphosis.
  • Examples: Petromyzon (lamprey) and Myxine (hagfish).
4.2.11.2 Class – Chondrichthyes
  • Marine animals with a streamlined body and cartilaginous endoskeleton.
  • The mouth is located ventrally.
  • Notochord is persistent throughout life.
  • Gill slits are separate and without operculum (gill cover).
  • The skin is tough, containing minute placoid scales.
  • Teeth are modified placoid scales directed backwardly.
  • Powerful jaws; predaceous animals.
  • Due to the absence of an air bladder, they have to swim constantly to avoid sinking.
  • The heart is two-chambered (one auricle and one ventricle).
  • Some have electric organs (e.g., Torpedo) and some possess a poison sting (e.g., Trygon).
  • Cold-blooded (poikilothermous) animals.
  • Sexes are separate; males have claspers on pelvic fins.
  • Internal fertilisation; many are viviparous.
  • Examples: Scoliodon (dogfish), Pristis (sawfish), Carcharodon (great white shark), Trygon (stingray).
4.2.11.3 Class – Osteichthyes
  • Includes both marine and freshwater fishes with a bony endoskeleton.
  • The body is streamlined.
  • The mouth is mostly terminal.
  • Have four pairs of gills covered by an operculum on each side.
  • The skin is covered with cycloid/ctenoid scales.
  • An air bladder is present, which regulates buoyancy.
  • The heart is two-chambered (one auricle and one ventricle).
  • Cold-blooded animals.
  • Sexes are separate.
  • Fertilisation is usually external.
  • Mostly oviparous and development is direct.
  • Examples:
    • Marine – Exocoetus (flying fish), Hippocampus (sea horse).
    • Freshwater – Labeo (rohu), Catla (katla), Clarias (magur).
    • Aquarium – Betta (fighting fish), Pterophyllum (angel fish).
4.2.11.4 Class – Amphibia
  • As the name indicates (Gr., Amphi: dual, bios, life), amphibians can live in aquatic as well as terrestrial habitats.
  • Most have two pairs of limbs.
  • The body is divisible into head and trunk; a tail may be present in some.
  • The amphibian skin is moist (without scales).
  • The eyes have eyelids; a tympanum represents the ear.
  • The alimentary canal, urinary, and reproductive tracts open into a common chamber called the cloaca, which opens to the exterior.
  • Respiration is by gills, lungs, and through the skin.
  • The heart is three-chambered (two auricles and one ventricle).
  • Cold-blooded animals.
  • Sexes are separate.
  • Fertilisation is external.
  • Oviparous and development is indirect.
  • Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless amphibia).
4.2.11.5 Class – Reptilia
  • The class name refers to their creeping or crawling mode of locomotion (Latin, repere or reptum, to creep or crawl).
  • They are mostly terrestrial animals, and their body is covered by dry and cornified skin, epidermal scales, or scutes.
  • They do not have external ear openings; the tympanum represents the ear.
  • Limbs, when present, are two pairs.
  • The heart is usually three-chambered, but four-chambered in crocodiles.
  • Reptiles are poikilotherms.
  • Snakes and lizards shed their scales as skin cast.
  • Sexes are separate.
  • Fertilisation is internal.
  • They are oviparous, and development is direct.
  • Examples:
    • Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), Calotes (garden lizard), Crocodilus (crocodile), Alligator (alligator).
    • Hemidactylus (wall lizard).
    • Poisonous snakes – Naja (cobra), Bangarus (krait), Vipera (viper).
4.2.11.6 Class – Aves
  • The characteristic features are the presence of feathers, and most can fly except flightless birds (e.g., ostrich).
  • Possess a beak.
  • The forelimbs are modified into wings.
  • The hind limbs generally have scales and are modified for walking, swimming, or clasping tree branches.
  • The skin is dry without glands except for the oil gland at the base of the tail.
  • The endoskeleton is fully ossified (bony), and the long bones are hollow with air cavities (pneumatic).
  • The digestive tract has additional chambers: the crop and gizzard.
  • The heart is completely four-chambered.
  • Warm-blooded (homoiothermous) animals.
  • Respiration is by lungs; air sacs connected to lungs supplement respiration.
  • Sexes are separate.
  • Fertilisation is internal.
  • Oviparous, and development is direct.
  • Examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), Neophron (vulture).
4.2.11.7 Class – Mammalia
  • Found in a variety of habitats – polar ice caps, deserts, mountains, forests, grasslands, and dark caves.
  • Some have adapted to fly or live in water.
  • The most unique characteristic is the presence of milk-producing glands (mammary glands) by which the young ones are nourished.
  • Have two pairs of limbs adapted for walking, running, climbing, burrowing, swimming, or flying.
  • The skin possesses hair.
  • External ears or pinnae are present.
  • Different types of teeth are present in the jaw.
  • The heart is four-chambered.
  • Homoiothermous.
  • Respiration is by lungs.
  • Sexes are separate, and fertilisation is internal.
  • Viviparous with few exceptions, and development is direct.
  • Examples:
    • Oviparous: Ornithorhynchus (platypus).
    • 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).