Animal Kingdom Classification
The Animal Kingdom includes all multicellular, eukaryotic, heterotrophic organisms that are classified based on various characteristics like body structure, mode of reproduction, level of organization, symmetry, etc.
The living world is full of a wide variety of animals. From microscopic organisms like protozoa to large animals like elephants and whales, the diversity is immense. To study and understand this diversity properly, biologists have grouped animals into various categories. This process of grouping animals based on their similarities is called classification.
Why Classification of Animals?
- It helps in the systematic study of animals.
- Makes it easier to identify and compare different organisms.
- Helps understand the evolutionary relationships among different species.
Basis of Classification
Animals are found in every corner of the world – from deep oceans to high mountains, from deserts to forests. Due to this great diversity, scientists need a systematic way to study and group them. This is why animals are classified based on their similar features.To classify animals correctly, biologists use specific characteristics. These characteristics are called the "basis of classification." They help in placing animals into different groups or phyla.
Why is Classification Needed?
- There are millions of animal species.
- Helps in identifying and studying animals easily.
- Shows relationships between different animals.
- Helps in understanding evolution.
Animals are classified based on various features. These are:
1. Level of Organisation
- Some animals are made of just cells (cellular level), like sponges.
- Others have tissues, organs, or organ systems, like humans.
2. Body Symmetry
- Asymmetrical: No symmetry (e.g., Sponge)
- Radial symmetry: Body divided into similar halves in any plane (e.g., Hydra)
- Bilateral symmetry: Only one plane divides the body into two equal halves (e.g., Human)
3. Germ Layers
- Number of embryonic layers formed during development.
- Diploblastic: 2 layers (ectoderm and endoderm)
- Triploblastic: 3 layers (ectoderm, mesoderm, and endoderm)
4. Body Cavity (Coelom)
- Acoelomate: No body cavity (e.g., Flatworms)
- Pseudocoelomate: False body cavity (e.g., Roundworms)
- Coelomate: True body cavity (e.g., Earthworms, Humans)
5. Segmentation
- Some animals have bodies divided into segments (e.g., Earthworms, Insects)
6. Notochord
- A flexible rod-like structure in some animals.
- Chordates have a notochord.
- Non-chordates do not have a notochord
The basis of classification provides the foundation for organizing animals into different groups. These features help us understand how animals are built, how they function, and how they are related to each other.
Major Groups of Animal Kingdom
The Animal Kingdom includes a huge variety of animals – from simple sponges to complex mammals like humans. To study this wide range of animals, scientists have divided them into major groups based on specific features like body structure, body cavity, symmetry, presence of notochord, and more.
The broadest division in the animal kingdom is between Non-Chordates and Chordates.
1. Non-Chordates – Animals without a notochord
These are the animals that do not have a notochord (a rod-like supporting structure found in chordates). Most of the animals in the animal kingdom are non-chordates. They may have simple or complex body structures.
Non-Chordate Phyla include:
- Porifera – Sponges
- Cnidaria – Jellyfish, Hydra
- Ctenophora – Comb jellies
- Platyhelminthes – Flatworms
- Nematoda – Roundworms
- Annelida – Earthworms, Leeches
- Arthropoda – Insects, Crabs, Spiders (largest group)
- Mollusca – Snails, Octopus
- Echinodermata – Starfish, Sea urchins
1. Phylum Porifera (Sponges)
Phylum Porifera includes the simplest multicellular animals commonly known as sponges. The name Porifera means "pore-bearing," referring to the tiny openings (ostia) all over their body through which water enters. These animals are aquatic, mostly marine, and are sessile, meaning they remain attached to a surface throughout their life.
Poriferans have a primitive body organization without true tissues or organs. Their body is designed to filter water for food and oxygen using a unique canal system. They exhibit asymmetry or radial symmetry and lack a nervous system. Despite their simple structure, sponges play an essential role in aquatic ecosystems as filter feeders.
- Body Organization: Cellular level
- Symmetry: Asymmetrical or Radial
- Body Cavity: Acoelomate
- Special Feature: Pores all over body, canal system for water flow
- Example: Sycon, Spongilla, Euspongia
2. Phylum Coelenterata / Cnidaria
Phylum Coelenterata, also called Cnidaria, includes aquatic animals that are radially symmetrical and diploblastic (having two germ layers – ectoderm and endoderm). These animals have a central body cavity called the coelenteron or gastrovascular cavity, hence the name Coelenterata.
A unique feature of this phylum is the presence of cnidocytes – specialized stinging cells found on the tentacles used for capturing prey and defense. Most cnidarians are marine, and they may live singly (e.g., Hydra) or in colonies (e.g., Corals).
They exhibit two body forms:
Polyp: cylindrical and sessile (e.g., Hydra, Sea anemone).
Medusa: umbrella-shaped and free-swimming (e.g., Jellyfish).
Polyp: cylindrical and sessile (e.g., Hydra, Sea anemone).
Medusa: umbrella-shaped and free-swimming (e.g., Jellyfish).
These organisms show tissue-level organization and have a simple nerve net. They reproduce both sexually and asexually, and many show alternation of generations (polyp ↔ medusa).
- Body Organization: Tissue level
- Symmetry: Radial
- Special Feature: Cnidoblasts (stinging cells), diploblastic
- Body Forms: Polyp and Medusa
- Example: Hydra, Jellyfish, Sea Anemone, Corals
3. Phylum Ctenophora (Comb jellies)
Phylum Ctenophora includes marine animals commonly known as sea walnuts or comb jellies. These are exclusively marine, free-swimming, and bioluminescent animals that often look jelly-like and transparent.
The name Ctenophora means "comb-bearing" because they have eight rows of ciliary plates (called ctenes) on their body, which help in locomotion. Unlike cnidarians, they do not have stinging cells (cnidocytes) but may have special sticky cells called colloblasts to capture prey.
- Body Organization: Tissue level
- Symmetry: Radial
- Special Feature: 8 rows of ciliary comb plates for locomotion, bioluminescent
- Example: Pleurobrachia, Ctenoplana
4. Phylum Platyhelminthes (Flatworms)
Phylum Platyhelminthes includes animals commonly known as flatworms. These animals are bilaterally symmetrical, triploblastic (three germ layers), and acoelomate (no body cavity). Their body is dorsoventrally flattened, giving them a flat, ribbon-like appearance.
They exhibit organ level of organization and are the first animals to have a head and central nervous system (cephalization). Many are parasitic, living inside the bodies of other animals, while a few are free-living (like Planaria in freshwater).
- Symmetry: Bilateral
- Germ Layers: Triploblastic
- Body Cavity: Acoelomate
- Special Feature: Dorsoventrally flattened body, parasitic
- Example: Planaria, Liver fluke, Tapeworm
5. Phylum Aschelminthes / Nematoda (Roundworms)
Phylum Aschelminthes (or Nemathelminthes) comprises a diverse assemblage of unsegmented, cylindrical worms that are mostly pseudocoelomate—they possess a fluid-filled body cavity (pseudocoel) between the gut and body wall, unlike flatworms (which are acoelomate). These animals are bilaterally symmetrical, triploblastic, and generally small—many are microscopic.
- Symmetry: Bilateral
- Body Cavity: Pseudocoelomate
- Special Feature: Unsegmented body, parasitic or free-living
- Example: Ascaris, Wuchereria, Ancylostom
6. Phylum Annelida (Segmented Worms)
Phylum Annelida includes animals commonly known as segmented worms. The name Annelida comes from the Latin word annulus, meaning "little ring", referring to the ring-like body segments present in these animals.
They are bilaterally symmetrical, triploblastic, and coelomate (true body cavity). Annelids are the first animals to show true segmentation (called metamerism) and have a well-developed organ-system level of organization.
They can be free-living or parasitic, and live in soil, freshwater, or marine habitats.
- Symmetry: Bilateral
- Coelom: True coelom (coelomate)
- Body Structure: Segmented body
- Example: Earthworm, Leech, Nereis
7. Phylum Arthropoda
Phylum Arthropoda is the largest phylum in the Animal Kingdom, including insects, spiders, crabs, centipedes, etc. The word Arthropoda means "jointed legs" (arthro = joint, poda = legs).
These animals have a segmented body, a hard exoskeleton made of chitin, and jointed appendages, which makes them highly mobile and adaptable. Arthropods are found in nearly every habitat – land, water, air, and even inside other organisms (as parasites).
- Symmetry: Bilatera
- Special Feature: Jointed appendages, chitinous exoskeleton
- Other Features: Open circulatory system, largest phylum in Animal Kingdom
- Example: Insects (Ants, Bees), Crabs, Spiders
8. Phylum Mollusca
Phylum Mollusca includes soft-bodied animals, many of which have a hard external shell. The word Mollusca comes from Latin mollis, meaning "soft." It is the second-largest phylum after Arthropoda and includes animals like snails, octopuses, oysters, and squids.
Molluscs are bilaterally symmetrical, triploblastic, and coelomate animals with a unsegmented body. Their body is typically divided into three parts: head, foot, and visceral mass. They have a mantle that secretes the shell (if present), and many have a rasping organ called a radula used for feeding.
- Body: Soft, unsegmented body with shell (usually)
- Body Parts: Head, foot, visceral mass
- Example: Snail, Octopus, Pila, Unio
9. Phylum Echinodermata
Phylum Echinodermata includes marine animals like starfish, sea urchins, and sea cucumbers. The word Echinodermata comes from Greek, where echinos means "spiny" and derma means "skin," referring to their spiny or rough outer body.
These animals are exclusively marine, triploblastic, coelomate, and show radial symmetry in adults (but bilateral in larvae). They have an endoskeleton made of calcium carbonate plates or spines.
A unique feature of echinoderms is the water vascular system, which helps in movement, feeding, respiration, and excretion using tube feet.
- Symmetry: Adults – radial, Larvae – bilateral
- Special Feature: Water vascular system, spiny skin
- Example: Starfish, Sea Urchin, Sea Cucumber
2. Chordates – Animals with a notochord
These animals have a notochord at some stage in their life. Most chordates also have a backbone and are more complex than non-chordates.
General Features:
Notochord
- Dorsal, hollow nerve cord
- Pharyngeal slits
- Post-anal tail
Subphylum 1: Urochordata (Tunicates)
- Notochord in larval stage only.
- Marine.
- Example: Ascidia, Salpa
Subphylum 2: Cephalochordata
- Notochord throughout life.
- Example: Amphioxus (Branchiostoma)
Subphylum 3: Vertebrata
- Notochord replaced by vertebral column.
- Advanced organ systems.
Classes under Vertebrata
1. Pisces (Fishes)
Pisces is a group of aquatic vertebrates that includes all types of fishes. They are cold-blooded (ectothermic) animals that live in freshwater or marine environments. Fishes are the first true vertebrates with a cranium, vertebral column, and jaws (in most types).
Their body is streamlined for swimming, and they breathe through gills. Most fishes have fins for movement and scales covering their body. Reproduction is mainly oviparous (egg-laying), and fertilization may be external or internal, depending on the species.
Pisces is broadly divided into two main classes:
1. Class Chondrichthyes – Cartilaginous fishes
2. Class Osteichthyes – Bony fishes
1. Class Chondrichthyes – Cartilaginous fishes
2. Class Osteichthyes – Bony fishes
Examples:
Chondrichthyes: Shark, Skate, Torpedo
Osteichthyes: Rohu, Catla, Hilsa, Sea horse
Chondrichthyes: Shark, Skate, Torpedo
Osteichthyes: Rohu, Catla, Hilsa, Sea horse
- Aquatic, gills, scales, fins.
- Cold-blooded (poikilothermic).
2. Amphibia
Amphibia is a class of vertebrates that includes animals like frogs, toads, salamanders, and newts. The name Amphibia comes from Greek, meaning "double life" (amphi = both, bios = life), because they can live both in water and on land.
Amphibians are cold-blooded (ectothermic) animals. They have a moist, smooth or rough skin (without scales) which is used for cutaneous (skin) respiration, along with lungs and gills (in some stages). Most amphibians lay eggs in water, and their development involves a larval stage (like tadpoles) that undergoes metamorphosis.
- Moist skin, both aquatic and terrestrial.
- Cold-blooded, gills or lungs.
- Example: Frog, Toad, Salamander
3. Reptilia
Reptilia includes cold-blooded, crawling vertebrates like lizards, snakes, turtles, crocodiles, and alligators. The word Reptilia comes from Latin "repere", meaning "to crawl" or "creep".
Reptiles are the first truly terrestrial vertebrates, adapted to life on land with dry, scaly skin to prevent water loss. They breathe only through lungs, and their body is covered with keratinized scales or scutes.
Reptiles are cold-blooded (ectothermic) and have a three-chambered heart (except crocodiles, which have four chambers). They lay shelled eggs on land (oviparous), and fertilization is internal.
- Dry, scaly skin
- Lay shelled eggs (cleidoic), lungs.
- Cold-blooded.
- Example: Lizard, Snake, Crocodile
4. Aves (Birds)
Aves is the class of vertebrates that includes all birds. The word Aves comes from Latin, meaning "birds". Birds are warm-blooded (endothermic) animals that are adapted for flight, though not all birds fly (e.g., ostrich, penguin).
They have a lightweight body, covered with feathers, and their forelimbs are modified into wings. Birds breathe through lungs with the help of air sacs, which makes respiration very efficient. They have a four-chambered heart, high metabolism, and lay hard-shelled eggs (oviparous).
- Feathers, wings, beak.
- Warm-blooded (homeothermic).
- Lay eggs, pneumatic bones.
- Example: Pigeon, Eagle, Parrot
5. Mammalia
Mammalia is the class of vertebrates that includes humans, dogs, cats, whales, bats, elephants, and many more. Mammals are warm-blooded (endothermic) animals that are highly evolved and found in all types of habitats—land, air, and water.
The defining feature of mammals is the presence of mammary glands, which produce milk to nourish their young. Their body is covered with hair or fur, and they have sweat and oil glands in the skin. Most mammals give birth to young ones (viviparous), though a few lay eggs.
They have a four-chambered heart, lungs for respiration, and a highly developed brain and nervous system.
- Hair, mammary glands.
- Give birth to young (mostly).
- Warm-blooded.
- Examples: Human, Cow, Lion, Whale
The major groups of the Animal Kingdom are divided mainly into non-chordates and chordates, based on the presence or absence of a notochord. This classification helps us to study animals in a more organized and scientific way.
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