CBSE Class 11 Biology Revision Notes Chapter 4 Animal Kingdom

Animal Kingdom explains how animals are classified using body organisation, symmetry, coelom, segmentation and notochord. In CBSE Class 11 Biology, this chapter covers non-chordates, chordates and major vertebrate classes with examples.

Animal Kingdom studies the diversity of animals and the features used to classify them. Since more than a million animal species have been described, classification helps place each animal group in a systematic position.

Use CBSE Class 11 Biology Revision Notes Chapter 4 for 2026–27 to revise the basis of classification, Porifera to Chordata, and vertebrate classes such as Pisces, Amphibia, Reptilia, Aves and Mammalia.

Key Takeaways

• Classification basis: Animals are grouped by organisation, symmetry, coelom, segmentation and notochord.
• Porifera to Echinodermata: These are major non-chordate phyla.
• Chordata: Chordates have notochord, dorsal hollow nerve cord and pharyngeal gill slits.
• Vertebrates: Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia are covered.

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Animal Kingdom Class 11 Biology Notes: Basis of Classification

Animals differ in structure and body form. Biologists classify them using fundamental features that remain useful across animal groups.

These features help identify relationships between animal phyla.

Levels of Organisation in Animal Kingdom Class 11 Notes

All animals are multicellular, but their cells are arranged at different levels of complexity.

Cellular Level of Organisation

In sponges, cells are arranged as loose aggregates. Some division of labour occurs among cells, but true tissues are absent.

Tissue Level of Organisation

In coelenterates, cells performing similar functions are arranged into tissues. This gives them a more organised body plan than sponges.

Organ Level of Organisation

In Platyhelminthes, tissues are grouped to form organs. Each organ performs a specific function.

Organ System Level of Organisation

In annelids, arthropods, molluscs, echinoderms and chordates, organs form systems. These systems perform functions such as digestion, circulation and reproduction.

Organ System Patterns in Class 11 Biology Chapter 4 Notes

Organ systems differ across animal groups. Two important patterns are digestive system and circulatory system.

Digestive System

A complete digestive system allows food intake and waste removal through separate openings.

Circulatory System

Closed circulation allows more controlled transport of substances.

Body Symmetry in Animal Kingdom Class 11 Biology Notes

Symmetry describes how an animal body can be divided into equal halves.

Bilateral symmetry is common in animals with active movement.

Diploblastic and Triploblastic Animals

Animals are grouped based on the number of embryonic germ layers.

In diploblastic animals, an undifferentiated layer called mesoglea is present between ectoderm and endoderm.

In triploblastic animals, mesoderm develops between ectoderm and endoderm. This allows formation of more complex organs.

Coelom in Animal Kingdom Classification

Coelom is the body cavity lined by mesoderm. It is an important basis for classification.

A true coelom provides space for internal organs and supports complex organisation.

Segmentation and Notochord in Class 11 Biology Chapter 4 Notes

Segmentation

In some animals, the body is divided externally and internally into repeated segments. This is called metameric segmentation or metamerism.

Earthworm is a common example of metameric segmentation.

Notochord

Notochord is a mesodermally derived rod-like structure present on the dorsal side during embryonic development.

Animals from Porifera to Echinodermata are non-chordates. Chordates have a notochord at least during some stage of life.

Phylum Porifera in Animal Kingdom Class 11 Notes

Members of Phylum Porifera are commonly called sponges.

Water enters through ostia, moves into the spongocoel and leaves through the osculum. This canal system helps in food gathering, respiration and waste removal.

Examples: Sycon, Spongilla, Euspongia

Phylum Coelenterata or Cnidaria in Animal Kingdom Notes

Coelenterates are aquatic, mostly marine animals. They may be sessile or free-swimming.

Cnidoblasts contain nematocysts and help in anchorage, defence and prey capture.

Polyp and Medusa

Some cnidarians show metagenesis. In Obelia, polyps produce medusae asexually, and medusae produce polyps sexually.

Examples: Physalia, Adamsia, Pennatula, Gorgonia, Meandrina

Phylum Ctenophora in CBSE Class 11 Biology Notes Chapter 4

Ctenophores are commonly called sea walnuts or comb jellies.

Bioluminescence is the ability of a living organism to emit light.

Examples: Pleurobrachia, Ctenoplana

Phylum Platyhelminthes in Biology Class 11 Chapter 4 Notes

Platyhelminthes are called flatworms because their body is dorsoventrally flattened.

Flame cells help in osmoregulation and excretion. Some members, such as Planaria, show high regeneration capacity.

Examples: Taenia, Fasciola, Planaria

Phylum Aschelminthes in Animal Kingdom Class 11 Biology Notes

Aschelminthes are called roundworms because their body is circular in cross-section.

Females are often longer than males.

Examples: Ascaris, Wuchereria, Ancylostoma

Phylum Annelida in Class 11 Biology Chapter 4 Notes

Annelids may be aquatic, terrestrial, free-living or parasitic.

Aquatic annelids such as Nereis have parapodia for swimming.

Examples: Nereis, Pheretima, Hirudinaria

Phylum Arthropoda in Animal Kingdom Class 11 Notes

Arthropoda is the largest phylum of Animalia and includes insects.

The body usually has head, thorax and abdomen. Respiratory organs may be gills, book gills, book lungs or tracheal system.

Phylum Mollusca in CBSE Class 11 Biology Revision Notes Chapter 4

Mollusca is the second largest animal phylum.

The mantle cavity contains feather-like gills, which help in respiration and excretion.

Examples: Pila, Pinctada, Sepia, Loligo, Octopus, Aplysia, Dentalium, Chaetopleura

Phylum Echinodermata in Animal Kingdom Class 11 Biology Notes

Echinoderms are spiny-bodied marine animals.

The water vascular system helps in locomotion, food capture, food transport and respiration.

Examples: Asterias, Echinus, Antedon, Cucumaria, Ophiura

Phylum Hemichordata in Class 11 Biology Chapter 4 Notes

Hemichordata was earlier placed under Chordata, but it is now treated as a separate non-chordate phylum.

Hemichordates have a stomochord in the collar region.

Examples: Balanoglossus, Saccoglossus

Phylum Chordata in Animal Kingdom Class 11 Biology Notes

Chordates are animals with key features such as notochord, dorsal hollow nerve cord and pharyngeal gill slits.

Chordates are bilaterally symmetrical, triploblastic, coelomate animals with organ system level of organisation.

Chordates and Non-Chordates Difference

Protochordates and Vertebrates in Animal Kingdom Notes

Phylum Chordata includes Urochordata, Cephalochordata and Vertebrata.

Urochordata and Cephalochordata are called protochordates and are exclusively marine.

All vertebrates are chordates, but all chordates are not vertebrates.

Classification of Vertebrates in CBSE Class 11 Biology Notes Chapter 4

Vertebrates are divided into Agnatha and Gnathostomata.

Class Cyclostomata in Animal Kingdom Class 11 Notes

Cyclostomes are jawless vertebrates and are ectoparasites on fishes.

After spawning, adults die. Larvae return to the ocean after metamorphosis.

Examples: Petromyzon, Myxine

Class Chondrichthyes in Class 11 Biology Chapter 4 Notes

Chondrichthyes includes cartilaginous fishes.

They must swim constantly to avoid sinking because air bladder is absent.

Examples: Scoliodon, Pristis, Carcharodon, Trygon

Class Osteichthyes in Biology Class 11 Chapter 4 Notes

Osteichthyes includes bony fishes found in marine and freshwater habitats.

Air bladder helps regulate buoyancy.

Class Amphibia in Animal Kingdom Class 11 Notes

Amphibians can live in water and on land.

Alimentary canal, urinary tract and reproductive tract open into a common chamber called cloaca.

Examples: Bufo, Rana, Hyla, Salamandra, Ichthyophis

Class Reptilia in CBSE Class 11 Biology Notes Chapter 4

Reptiles are mostly terrestrial animals.

Snakes and lizards shed their scales as skin cast.

Examples: Chelone, Testudo, Chameleon, Calotes, Crocodilus, Alligator, Hemidactylus, Naja, Bangarus, Vipera

Class Aves in Class 11 Biology Chapter 4 Notes

Aves includes birds. Their forelimbs are modified into wings.

Most birds can fly, but flightless birds such as ostrich are exceptions.

Examples: Corvus, Columba, Psittacula, Struthio, Pavo, Aptenodytes, Neophron

Class Mammalia in Animal Kingdom Class 11 Biology Notes

Mammals are found in polar regions, deserts, mountains, forests, grasslands, caves and aquatic habitats.

Mammary glands produce milk to nourish young ones.

Quick Comparison of Major Animal Phyla

Important Terms from Animal Kingdom Class 11 Notes

NCERT-Based Exam Points

• Sponges show cellular level of organisation.
• Coelenterates show tissue level of organisation.
• Platyhelminthes show organ level of organisation.
• Annelids, arthropods, molluscs, echinoderms and chordates show organ system level.
• Coelenterates, ctenophores and echinoderms show radial symmetry.
• Annelids and arthropods show bilateral symmetry.
• Coelenterates are diploblastic.
• Platyhelminthes to chordates are triploblastic.
• Platyhelminthes are acoelomate.
• Aschelminthes are pseudocoelomate.
• Annelida to Chordata are coelomate.
• Earthworm shows metameric segmentation.
• Porifera has a canal system.
• Cnidaria has cnidoblasts.
• Ctenophora has comb plates.
• Platyhelminthes has flame cells.
• Aschelminthes has a complete alimentary canal.
• Annelida has nephridia.
• Arthropoda has jointed appendages.
• Mollusca has radula.
• Echinodermata has water vascular system.
• Hemichordata has proboscis, collar and trunk.
• Chordata has notochord, dorsal hollow nerve cord and pharyngeal gill slits.
• All vertebrates are chordates, but all chordates are not vertebrates.
• Aves and Mammalia are warm-blooded.
• Fishes, amphibians and reptiles are cold-blooded.

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