Animal Kingdom Definition, Characteristics, Phyla, Examples

An animal phylum is a broad taxonomic category that encompasses a group of species with a shared evolutionary lineage. The classification of animal phyla is primarily based on common ancestry, and it also considers the collective body plan characterized by unique morphological features.

Characteristics of Animal Kingdom

  1. Early Developmental Stages:

The foundational features of animal phyla are evident in early developmental processes like gastrulation. These processes provide a basis for subsequent development.

  1. Embryonic Tissue Layers:

Most animals, with the exception of Porifera and Cnidaria, exhibit triploblastic growth, meaning they develop three embryonic tissue layers during gastrulation. These layers give rise to various cell types in mature animals.

  1. Symmetry:

    • Bilateral Symmetry: Animals with bilateral symmetry have distinct front and back sides, with roughly mirrored left and right sides. For example, humans exhibit bilateral symmetry.
    • Radial Symmetry: Creatures with radial symmetry are more or less spherical and lack distinct left or right sides. They can move in any direction with equal ease. This is characteristic of organisms like Cnidarians.
    • Asymmetry: Some animals, like sponges, lack a specific symmetry.
  2. Cephalization:

Cephalization refers to the presence of a head in organisms. While it’s absent in organisms like cnidarians (e.g., jellyfish), bilaterally symmetrical animals, including humans, exhibit cephalization.

  1. Body Cavity:

    • Coelomate: These animals have internal organs enclosed in a cavity (coelom) entirely lined with mesodermal tissue. Arthropods and chordates fall into this category.
    • Pseudocoelomate: Internal organs develop within a coelom that is partially lined with mesodermal tissue. Worms are examples of pseudocoelomates.
    • Acoelomate: These organisms lack cavities and have a simple body plan.
  2. Segmentation:

Many animals, such as those in phyla Annelida, Arthropoda, and Chordata, exhibit segmentation. Conversely, phyla Cnidaria are unsegmented.

  1. Digestive Tract:

All animal phyla, except sponges, possess a digestive system. Flatworms and cnidarians have a digestive tract with only one opening, while organisms with a complete digestive tract start from the mouth and end at the anus.

  1. Circulatory System:

Some organisms have a closed circulatory system, while others have an open system.

  1. Exoskeleton:

Arthropods, like insects and crustaceans, possess a tough external exoskeleton for protection. They undergo molting (ecdysis) to shed their exoskeletons and grow. Nematodes also molt, but they have more flexible exoskeletons, known as cuticles.

Different phyla of Animal Kingdom

The Animal Kingdom is incredibly diverse and is classified into several major phyla based on shared characteristics and evolutionary relationships. Some of the major phyla within the Animal Kingdom:

  1. Porifera (Sponges):

    • Simple, multicellular organisms with porous bodies.
    • Lack true tissues and organs.
    • Filter-feeders that rely on water flow to extract nutrients.
  2. Cnidaria (Corals, Jellyfish, Sea Anemones):
    • Radially symmetrical animals with stinging cells called cnidocytes.
    • Possess a simple nerve net but lack a centralized nervous system.
    • Exhibit two tissue layers (diploblastic).
  3. Platyhelminthes (Flatworms):
    • Flat, unsegmented worms with bilateral symmetry.
    • Possess a central nervous system with simple brains.
    • Many are parasitic, while others are free-living.
  4. Nematoda (Roundworms):
    • Unsegmented worms with cylindrical bodies.
    • Complete digestive tract with a separate mouth and anus.
    • Some are parasitic, while others are free-living.
  5. Mollusca (Mollusks):
    • Soft-bodied animals often protected by a hard shell.
    • Include snails, clams, octopuses, and squids.
    • Radula (feeding organ) is characteristic of this phylum.
  6. Annelida (Segmented Worms):
    • Segmented body with repeated units (segments).
    • Possess a true coelom (body cavity).
    • Include earthworms, leeches, and marine worms.
  7. Arthropoda (Arthropods):
    • Largest phylum with jointed legs and an exoskeleton.
    • Include insects, spiders, crustaceans, and arachnids.
    • Highly diverse and successful group.
  8. Echinodermata (Echinoderms):
    • Marine animals with spiny skin and a unique water vascular system.
    • Include starfish, sea urchins, and sea cucumbers.
    • Exhibit radial symmetry as adults.
  9. Chordata (Chordates):
    • Include vertebrates (animals with a backbone) and some non-vertebrate groups.
    • Possess a notochord, dorsal nerve cord, pharyngeal slits, and a post-anal tail at some stage of their life cycle.
  10. Mammalia (Mammals):
    • Vertebrates characterized by mammary glands, hair or fur, and live birth.
    • Highly developed nervous system and complex behavior.
  11. Aves (Birds):
    • Warm-blooded vertebrates with feathers, beaks, and lay hard-shelled eggs.
    • Highly adapted for flight, although some species are flightless.
  12. Reptilia (Reptiles):
    • Cold-blooded vertebrates with scaly skin and lay amniotic eggs.
    • Include snakes, lizards, turtles, and crocodiles.
  13. Amphibia (Amphibians):

    • Cold-blooded vertebrates with moist skin and undergo metamorphosis.
    • Typically have a dual life cycle, starting in water as larvae and transitioning to land as adults.

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