Animal Kingdom Classification: Phyla, Subphyla, and Their Unique Features

The animal kingdom is a large and diverse group of living organisms that are multicellular and eukaryotic. Unlike plants, animals do not have cell walls or chlorophyll, which means they cannot make their own food through photosynthesis. Instead, they rely on various heterotrophic modes of nutrition.

Scientists classify the animal kingdom into different phyla based on characteristics such as body structure, presence or absence of a true body cavity, type of symmetry, and more. This animal kingdom classification helps us organise the vast diversity of animals into manageable groups.

Classification of the Animal Kingdom

The animal kingdom is generally divided into ten major phyla:

1. Porifera

2. Coelenterata (Cnidaria)

3. Platyhelminthes

4. Nematoda

5. Annelida

6. Arthropoda

7. Mollusca

8. Echinodermata

9. Hemichordata

10. Chordata

A quick mnemonic to remember these phyla is:

“Po Co Pla Ni An Ar Mo Ec He Cho”

• Po – Porifera

• Co – Coelenterata

• Pla – Platyhelminthes

• Ni – Nematoda

• An – Annelida

• Ar – Arthropoda

• Mo – Mollusca

• Ec – Echinodermata

• He – Hemichordata

• Cho – Chordata

Below is a detailed classification of animals within each phylum, including key features and examples.

Phylum Porifera

• Key Features:

• Commonly called sponges.

• They have a porous body with numerous tiny openings or pores.

• They are multicellular but do not have well-defined tissues or organs.

• A canal system formed by these pores aids in the circulation of water and nutrients.

• Mostly found in marine habitats, and they are non-motile.

• Examples: Spongilla, Sycon

Did You Know?

Sponges can regenerate if broken into smaller pieces, as each piece can grow into a new sponge.

Phylum Coelenterata (Cnidaria)

• Key Features:

• Body contains a hollow cavity.

• Tissues are arranged in two layers (diploblastic).

• Can exist in colonies (like corals) or live solitarily (like sea anemones).

• They have special stinging structures called nematocysts for defence and capturing prey.

• Mostly aquatic, found in marine and sometimes freshwater habitats.

• Examples: Hydra, Jellyfish

Fun Fact

Corals, though appearing like colourful plants, are actually colonies of tiny animals called polyps.

Phylum Platyhelminthes

• Key Features:

• Commonly known as flatworms.

• They are dorsoventrally flattened (thin from top to bottom).

• Tissues are triploblastic (three layers), but they lack a true coelom (body cavity).

• Bilaterally symmetrical.

• They can be free-living (Planaria) or parasitic (liver flukes).

• Examples: Tapeworm, Planaria

Interesting Note

Planaria can regenerate a complete worm from small body segments, making them popular in regeneration studies.

Phylum Nematoda

• Key Features:

• Also called roundworms.

• Triploblastic and bilaterally symmetrical.

• Possess a pseudocoelom (false body cavity).

• Many nematodes are parasitic and can cause diseases in humans and other organisms (for example, Ascaris causes ascariasis).

• Examples: Ascaris (roundworm), Wuchereria (filarial worm)

Health Tip

Washing fruits and vegetables thoroughly helps prevent nematode infections.

Phylum Annelida

• Key Features:

• Commonly known as segmented or ringed worms.

• Body is segmented and shows bilateral symmetry.

• Triploblastic with a true body cavity (coelom).

• They can be found in marine environments, freshwater, or on land (moist soil).

• Have well-developed organ systems.

• Examples: Earthworm, Leech

Useful Fact

Earthworms are often called “friends of farmers” because they help in aerating and fertilising the soil.

Phylum Arthropoda

• Key Features:

• This is the largest phylum in the animal kingdom.

• Name means “jointed legs”.

• Bilaterally symmetrical, triploblastic, with a segmented body.

• Have an exoskeleton made of chitin.

• They possess an open circulatory system (blood does not flow in closed vessels).

• Examples: Spiders, Butterflies, Mosquitoes, Cockroaches

Fun Observation

Arthropods can be found in every possible habitat – from deep seas to high mountains.

Phylum Mollusca

• Key Features:

• Second-largest phylum in the animal kingdom.

• Bilaterally symmetrical, triploblastic, and less segmented than annelids and arthropods.

• Possess a well-developed organ system and generally have an open circulatory system.

• Many have a muscular foot for movement and a mantle that secretes a shell.

• Examples: Snails, Octopus, Clams

Extra Info

Octopuses are among the most intelligent invertebrates, capable of problem-solving.

Phylum Echinodermata

• Key Features:

• Means “spiny-skinned” animals (echino = spines, derma = skin).

• They are exclusively marine.

• Adult echinoderms have radial symmetry, but their larvae show bilateral symmetry.

• Triploblastic and have a true coelom.

• Have a water vascular system that helps in movement, feeding, and respiration.

• Examples: Sea Urchins, Starfish, Brittle Stars

Cool Fact

Starfish can regenerate lost arms, which can sometimes lead to a completely new individual arm regrowing into a full starfish.

Phylum Hemichordata

• Key Features:

• Worm-like marine animals with a soft, fragile body.

• The body is divided into a proboscis, collar, and trunk.

• Organ-system level of organisation, with an open circulatory system.

• Respiration occurs through gill slits.

• Sexes are usually separate, and fertilisation is external.

• Examples: Balanoglossus, Saccoglossus

Note

Hemichordates are sometimes considered a link between invertebrates and chordates, due to certain similar features.

Phylum Chordata and Its Subphyla

• Key Features of Chordates:

• Bilaterally symmetrical, triploblastic.

• Possess a notochord (in some stage of life) and a dorsal nerve cord.

• Closed circulatory system.

Phylum Chordata is further divided into three subphyla:

1. Urochordata (Tunicates)

• Notochord is present only in the larval stage.

• Adults are mostly sessile (non-motile).

2. Cephalochordata (Lancelets)

• Notochord is present throughout life.

• The body is fish-like, and they live in marine habitats.

3. Vertebrata

• Have a vertebral column (backbone) replacing the notochord in adults.

• Further classified into classes such as Pisces (fishes), Amphibia, Reptilia, Aves (birds), and Mammalia (mammals).

Quick Quiz (With Answers)

1. Which phylum is known for spiny skin?

Answer: Echinodermata

2. Name the largest phylum in the animal kingdom.

Answer: Arthropoda

3. Which phylum includes roundworms?

Answer: Nematoda

4. Which phylum is characterised by a water vascular system?

Answer: Echinodermata

5. In which phylum do animals have a pseudocoelom?

Answer: Nematoda

Related Topics

• Invertebrates

• Multicellular Organisms

• Chordata