The Animal Kingdom includes a vast diversity of organisms ranging from simple sponges to complex mammals. Animals are:
- Multicellular
- Eukaryotic
- Heterotrophic
- Usually motile
They show different levels of organization, symmetry, body plans, and evolutionary adaptations.
Basis of Classification
Animals are classified based on several fundamental features:
1. Levels of Organization
- Cellular level: Cells loosely arranged (e.g., Porifera)
- Tissue level: Cells organized into tissues (e.g., Cnidaria)
- Organ level: Organs present (e.g., Platyhelminthes)
- Organ system level: Complex systems (e.g., Chordates)
2. Body Symmetry
- Asymmetrical: No symmetry (e.g., Sponges)
- Radial symmetry: Divisible into equal halves in many planes (e.g., Cnidaria)
- Bilateral symmetry: Divisible into two equal halves (e.g., humans)
3. Germ Layers
- Diploblastic: Two layers (ectoderm + endoderm)
- Triploblastic: Three layers (ectoderm, mesoderm, endoderm)
4. Coelom (Body Cavity)
- Acoelomate: No body cavity
- Pseudocoelomate: False cavity
- Coelomate: True cavity
5. Segmentation
- Metamerism: Repetition of body segments (e.g., earthworm)
6. Notochord
- Present in chordates
- Absent in non-chordates
Classification of Animal Kingdom
Animals are divided into:
- Non-Chordates
- Chordates
Non-Chordates
1. Phylum Porifera (Sponges)
Characteristics:
- Cellular level organization
- Asymmetrical
- Body with pores (ostia)
- Canal system present
- Skeleton made of spicules
Examples:
- Sycon
- Spongilla
2. Phylum Cnidaria (Coelenterata)
Characteristics:
- Radial symmetry
- Diploblastic
- Tissue level organization
- Presence of cnidoblasts (stinging cells)
- Gastrovascular cavity
Examples:
- Hydra
- Jellyfish
3. Phylum Ctenophora
Characteristics:
- Marine organisms
- Bioluminescent
- Comb plates for locomotion
Example:
- Pleurobrachia
4. Phylum Platyhelminthes (Flatworms)
Characteristics:
- Bilateral symmetry
- Triploblastic
- Acoelomate
- Flattened body
Examples:
- Planaria
- Tapeworm
5. Phylum Nematoda (Aschelminthes)
Characteristics:
- Cylindrical body
- Pseudocoelomate
- Complete digestive system
Examples:
- Ascaris
- Wuchereria
6. Phylum Annelida
Characteristics:
- Segmented body
- True coelom
- Closed circulatory system
Examples:
- Earthworm
- Leech
7. Phylum Arthropoda
Characteristics:
- Jointed appendages
- Exoskeleton made of chitin
- Open circulatory system
- Largest phylum
Examples:
- Cockroach
- Butterfly
- Crab
8. Phylum Mollusca
Characteristics:
- Soft body
- Muscular foot
- Mantle present
- Shell present in many
Examples:
- Snail
- Octopus
9. Phylum Echinodermata
Characteristics:
- Marine
- Radial symmetry (adults)
- Water vascular system
- Spiny skin
Examples:
- Starfish
- Sea urchin
Chordates
General Characteristics
- Notochord present
- Dorsal hollow nerve cord
- Pharyngeal gill slits
- Post-anal tail
Classification of Chordates
1. Subphylum Urochordata
- Notochord present only in larval stage
Example: Ascidia
2. Subphylum Cephalochordata
- Notochord extends throughout life
Example: Amphioxus
3. Subphylum Vertebrata
- Backbone present
- Advanced organ systems
Classes of Vertebrata
1. Class Cyclostomata
- Jawless fishes
- Parasitic
Example: Lamprey
2. Class Pisces (Fishes)
Characteristics:
- Aquatic
- Gills for respiration
- Fins for locomotion
Types:
- Cartilaginous (Sharks)
- Bony (Rohu)
3. Class Amphibia
Characteristics:
- Live on land and water
- Moist skin
- External fertilization
Example: Frog
4. Class Reptilia
Characteristics:
- Dry, scaly skin
- Internal fertilization
- Cold-blooded
Example: Snake
5. Class Aves (Birds)
Characteristics:
- Feathers
- Wings
- Warm-blooded
Example: Pigeon
6. Class Mammalia
Characteristics:
- Hair on body
- Mammary glands
- Warm-blooded
Example: Human
Differences Between Chordates and Non-Chordates
| Feature | Chordates | Non-Chordates |
|---|---|---|
| Notochord | Present | Absent |
| Nerve cord | Dorsal | Ventral |
| Circulatory system | Closed | Mostly open |
Important NEET & CBSE Points
- Largest phylum: Arthropoda
- Only phylum with water vascular system: Echinodermata
- Notochord present in: Chordates
- Diploblastic animals: Cnidaria
- Segmentation seen in: Annelida
Quick Revision Notes
- Porifera → pores
- Cnidaria → stinging cells
- Arthropoda → jointed legs
- Mollusca → soft body
- Chordata → notochord
Frequently Asked Questions (FAQs)
Q1. What is the basis of classification?
Level of organization, symmetry, coelom, notochord.
Q2. Which is the largest phylum?
Arthropoda.
Q3. What is notochord?
Flexible rod supporting body in chordates.
Q4. Which animals are diploblastic?
Cnidarians.
Conclusion
The Animal Kingdom shows immense diversity and complexity. Understanding classification helps in studying evolutionary relationships and structural differences. This chapter is highly important for CBSE board exams and NEET preparation.