Kingdom Animalia


Kingdom Animalia

Animals are

  • eukaryotic
  • multicellular organisms
  • that do not possess a cell wall
  • heterotrophs, digest internally
  • most are capable of locomotion

Various phyla:

  • Cnidaria
  • Platyhelminthes
  • Annelida
  • Anthropoda
  • Chordata
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Phylum Cnidaria

  • include marine jellyfish and freshwater Hydra
  • radially symetrical
  • restricted to aqueous medium for support + as have no means of restricting water loss
  • hydrostatic skeleton formed by the fluid filled enteron (gut cavity)
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Phylum Platyhelminthes

Flatworms eg. planarians and river flukes

  • Structure:
    • bilaterally symmetrical
    • Flattened dorso-ventrally

An advantage of bilateral symmetry is animals have a 'front' where sensory perceptors can be positioned, so they can 'test' an environment before entering. Streamlined bilaterally symmetrical shape makes movement much easier

  • much more tissue than cnidarians
  • tube-like gut rather than enteron
    • meaning solid tissue fills the space btwn outer surface and gut
      • this tissue allows specialisation
      • also means many cells per unit volume involved in metabolic activity, meaning high demands for O2
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Phylum Platyhelminthes

  • Dorso-ventral flattening increases SA:Vol ratio
    • advantages:
      • increases uptake of O2 due to high SA:Vol ratio
      • decreases diffusion distance from body surface to body cells
  • Platyhelminthes have a single opening to the gut
    • means the remains of food are passed out of same body opening through which it enters
  • Surrounding aqueous medium provides suppt for most platyhelminthes, but cells btwn outer surface and gut provide support as 'packing tissue'
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Radial vs Bilateral Symmetry


  • radial symmetry means there is symmetry around a central axis
  • no left or right 'side'
  • body can be divided in half by any plane through the cross section ie. organism is round in cross-section
  • Sessile animals show radial symmetry, allowing them to respond to environment in all directions


  • Bilateral symmetry means that the body can be divided into two equal halves on each side of a central axis- there are left and right 'sides'
  • The two halves are mirror images of each other
  • Associated with animals that move through their environment to search for food
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Phylum Annelida

  • round worms eg. earthworms and lugworms
  • more rounded in cross-section compared to platyhelminthes
  • Bilaterally similar
  • Have a coelom - spaces within the tissue that lies btwn body surface and the gut
  • Chatae- external bristles of chitin that help w/ ease of mvmt
  • Cylindrical shape provides large SA:Vol ratio
    • Earthworms have a fairly low level of activity ∴ low rate of O2 consumption, so don't need special organs for gas exchange
  • Advantages of possessing a coelom
    • ratio of SA:metabolically active tissue is increased
      • why they're round- not same need to maximise SA:vol ratio
    • can function as effective hydroskeleton, as fluid filled
    • muscles involved in locomotion separated from gut muscles
      • allows mvmt of organism and peristaltic gut mvmts to occur independently
    • provides room for development of organs
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Phylum Annelida

  • metamerically segmented- body is divided into large no. of structurally similar segments, each w/ own body cavity
  • Annelids have a blood system and simple nervous system
  • One-way gut w/ separate mouth and anus
    • allows regional specialisation
    • prevents food waste being mixed up w/ incoming food
    • w/in the gut there is 
      • a muscular pharynx
      • an oesophagus
      • a crop (storage area)
      • a muscular gizzard (for mechanical absorption)
      • an intestine for absorption
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Phylum Anthropoda

eg. insects and spiders

  • bilaterally symmetrical
  • typically have fixed no. of metameric segments in each region of body eg. head, thorax and abdomen for insects - 3 in rhorax, 10-11 in abdomen
  • jointed limbs
  • both a mouth and an anus
  • gut shows regional specialisation


  • bilaterally similar and have fixed no. of metameric segments,
  • however have 4 pairs of legs rather than 3
  • body has 2 main sections rather than 3 (head + thorax combined - cephalothorax)
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Phylum Anthropoda

  • Anthropods, particularly insects are most successful animal group on earth
  • as basic insect body plan can be easily adapted evolutionarily to fill wide range of niches
    • eg. mouthparts evolved,
    • have wings,
    • several distinct body forms (like larval) w/ separate food sources
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Phylum Chordata

  • Vertebrates are chordates
  • main groups are the fish, amphibians, reptiles and mammals
  • bilaterally symmetrical and segmented
  • body cavity much greater proportionally in vertebrates
    • + more continuous than in annelids
    • contains much more extensively developed and complex organs organised into complex systems eg. digestive, circulatory
  • there is a vertebral (spinal) column
    • w/ segmented muscle blocks
    • w/ an endoskeleton consisting of an internal jointed system of calcified bones
  •  One-way gut w/ a mouth and anus
    • high degree of regional specialisation along the length of the gut
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Phylum Chordata

Note: segmentation is a linear series of repeating units

most obvious in annelids and arthropods

more subtle in chordates, includes vertebrae and rib bones

NOTE: there are a very small group of non-vertebrate chordates eg. lancelets

Evolutionarily they are a bridge btwn invertebrates and vertebrates, with a stiff dorsal rod rather than a true backbone

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Evolutionary trends in the Kingdom

The sequence from Cnidarians→Platyhelminthes→Annelida→Arthropoda→Chordata shows no. of evolutionary trends including:

  • Gradiation from radial to bilateral symmetry
  • Gradiation from solid tissue btwn body surface and the gut lining to the presence of cavities (that reduce the amnt of metabolically active tissue per unit volume)
  • Development of a one-way gut as opposed to a gut cavity (cnidarians) or gut with only one opening (platyhelminthes)
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