Classification of Animal Kingdom

by Vinodh Reddy


  • When any plane passing through the central axis of the body divides the organism in two halves that are approximately mirror images it is called Radial symmetry and the animals showing radial symmetry are called Radiata.
  • When the body can be divided into identical left and right halves in only one plane. This kind of symmetry is called bilateral symmetry and such animals are called Bilateria.
  • Almost 99 percent of animals are invertebrates (animals without backbone) and the remaining represents the vertebrates (animals with backbone). Also, the animals are categorised into two major groups, non-chordates and chordates, on the basis of the presence or absence of notochord at some stage in their life.
  • The animal kingdom is divided into 35 Phyla (singular:Phylum) of which 11 are considered as major Phyla.

1. Phylum Protozoa (Unicellular Protist Animals)

  • Phylum Protozoa are microscopic organisms in which a single cell performs all the vital activities.
  • They are aquatic (fresh water and marine) and cosmpolitan in distribution. Some forms are parasitic. The protozoan cell body is either naked, (for example, amobea) or surrounded by a non-rigid pellicle (cellulose is absent in pellicle).
  • Different types of locomotory organs are found in protozoans.
  • Most protozoans are free-living and aquatic. They are holozoic and feed largely on bacteria, microscopic algae and minute animals such as rotifiers including members of t6heir own species. Some protozoans are holophytic i.e. they prepare their own food by photosynthesis (e.g. Euglena). The parasitic protozoans feed on materials obtained from the hosts (e.g. Monocystsis).
    1. Free living: Euglena, Amobea, Paramoecium, Noctiluca and Elphidium.
    2. Parasitic: Monocystis, entamoeba, Plasmodium. Trypanosma and Giardia.

2. Phylum Porifera (Pore Bearing Animals)

  • Phylum Porifera known as sponges. They are the most primitive group of multicellular animals. About 5000 species of sponges are known. Most of them are marine and remain attached to rocks. A few live in fresh water.
  • The sponges are diploblastic.
  • Sponges reproduce asexually by fragmentation.

3. Phylum Cnidaria

  • phylum cnidaria name is derived from the stinging cells or cnidoblasts present on the ectoderm of tentacles and body of the carnivorous animals. cnidarians have achieved tissue grade of organization and they exhibit a blind sac body plan and radial symmetry.
  • Cnidarians are diplosblastic animals in which the body wall consists of only two layers or cells, an outer ectoderm and an inner ectoderm, separated by a gelatinous layer of mesoglea.
  • Examples: hydra, Obelia, porpita, vellela, Physalia (portuguese man of war), Aurelia (jellyfish), Adamsia (Sea anemone), Pennatula (Sea-pen) and Gorgonia (Sea-fan).

4. Phylum Ctenophora

  • Ctenophores are marine animals with transparent and flat or oval body shape. Polyphase is absent in their life cycle. These are bilaterally symmetrical and devoid of cnidoblast cells.
  • The presence of a special sense organ at the opposite end of the mouth (aboral end) is the characteristic of the mouth (aboral end) is the characteristic of the membes of this phylum. They produce only by sexual means and do not exhibit larval phase in their life cycle.

5. Phylum Platyhelminthes (Flatworms)

  • Phylum platyhelminthes are dorsoventrally flattened and, hence, commonly known as flatworms. these are mostly parasites.
  • These are triploblastic and unsegmented animals exhibiting bilateral symmetry.
  • They produce both sexually and asexually.
  • They are hermaphrodites or bisexual i.e., both male and female sex cells are produced by the same individual.
  • Examples: Taenia (Tapeworm), Fasciola (Liver Fluke), Schistosoma (Blood Fluke) etc.

6. Phylum Nemathelminthes (Round Worms)

  • These are also known as nematodes. Their bodies appear circular in cross-section, hence, the roundworm. Though not apparent they are possibly the most abundant and numerous among animals.
  • Round worms are bilaterally symmetrical, triploblastic and pseudocoelomate animals with an organ system grade of organization.
  • Round worms have a tube within a tube body plan with mouth, faringes, intestine and anus.
  • Sexes are usually separate, often with small male and large female individuals.
  • Examples: Ascaris, Wuchereria (Filaria worm), Ancylostoma (Hook worm), Enterobius (Pin worm) and Rhaditis.

7. Phylum Annelida

  • Phylum annelida metamerically segemented animals with a true coelom.
  • Examples: Nereis, Aphrodite (sea mouse), Pheretima (earthworm), Tubifex, Hirudinaria (leech), Chaetopterus, Erebella and Bonnellia.

8. Phylum Mollusca (Soft Bodies Animals)

  • Phylum mollusca are triploblastic coelomates and usually with bilateral symmetry. They are terrestrial, marine and fresh water inhabitants.
  • Examples: Pilla (apple snail), Achatina (land snail), Lamellidens (mussel), Pinctada (peal oyster), Sepia (cutlefish), lologo (squid0, Octopus (devilfish), Doris (sea-lemon), Aplysia (sea-hare) and Teredo (shipworm).

9. Phylum Arthropoda

  • The phylum arthropoda constitutes the largest group of animals with about 900,000 species. These are triploblastic, coelomate and bilaterally symmetrical animals.
  • Arthropoda have a segmented body, eachsegment bearing a pair of jointed appenadages covered by a joint of exoskeleton.
  • Arthropoda are unisexual.
  • Examples: Araneus (garden spider), limulus (king crab), Buthus (scorpion), Eupagurus (hermit crab), Cancer (common crab), Macrobrachium (prawn), Lepisma (silverfish), periplaneta (cockroach), Apis (bee), anopheles (mosquito), Musca (housefly), Leptocorisa (paddy pest: gandhi poka), Triops (tadpole fish), Daphnia (water flea), Cyclops, Squilla, Astacus (crayfish), Lepas and Balanus (Barnacle).

h3 style=”text-align: left;”>10. Phylum Chordata (Scordates)

  • Chordata refers to the group of animals which possess notochord either throughout or during early embryonic life. Notochord is a stiff and flexible rod of tissues lying ventral to nerve chord. All the chordates are triploblastic, coelomate and bilaterally symmetrical.
  • Phylum Chordata is divided into four subphyla viz. subphylum Hemichordata or stomochordata, subphylum Urochordata or Tunicate, subphylum Cephalochordata or Acrania and subphylum Vertebrata.

11. Phylum Echinodermata (Spiny Skinnerd Animals)

  • Phylum Echinoderms bears many spines and hence all are marine, triploblastic and coelomate.
  • The most distinctive of water vascular system, which is a part of the coelom. Its main function is locomotion and capture of food.
  • Examples: Asterias (starfish or sea star), echinus (sea urchin), Echinocardium (heart urchin), Antedon (feature star of sea lily), Cucunaria (sea cucumber) and Ophitura (brittle star).