Board exam focus — Animal Kingdom (CBSE & HBSE)

This chapter establishes the framework for classifying the diverse animal kingdom using fundamental criteria such as levels of organisation, body symmetry, germ layers, presence or absence of a coelom, segmentation, and the notochord. It then surveys the non-chordate phyla from Porifera to Echinodermata and the chordates from Protochordata to Mammalia, with characteristic features and representative examples for each group.

Basis of Classification

Why Classify Animals?

More than a million described animal species are arranged not by superficial resemblance but by fundamental features that reflect evolutionary relationships. Common features such as cell arrangement, body symmetry, the nature of the coelom, and patterns of digestive, circulatory and reproductive systems form the basis of classification.

1. Levels of Organisation

NEET trap: A tissue level of organisation first appears in Coelenterata (Cnidaria), not Porifera.

2. Symmetry

• Asymmetrical — no plane divides the body into equal halves (e.g., most sponges).
• Radial symmetry — any plane through the central axis gives two equal halves (e.g., Coelenterata, Ctenophora, adult Echinodermata).
• Bilateral symmetry — only one plane (the sagittal plane) gives two identical halves (e.g., Annelida, Arthropoda, Chordata).

3. Diploblastic and Triploblastic Organisation

• Diploblastic: two germ layers — ectoderm and endoderm — with an undifferentiated mesoglea in between (Coelenterata).
• Triploblastic: three germ layers — ectoderm, mesoderm and endoderm (Platyhelminthes to Chordata).

4. Coelom

The coelom is a body cavity lined by mesoderm.

5. Segmentation and Notochord

• Metameric segmentation: body externally and internally divided into serially repeated segments (metameres), e.g., earthworm.
• Notochord: a mesodermally derived rod formed on the dorsal side during embryonic development. Animals with a notochord are chordates; those without are non-chordates.

Classification of Non-Chordates (Porifera to Echinodermata)

Phylum Porifera (Sponges)

• Primitive multicellular animals with cellular level of organisation.
• Possess water-transport (canal) system; water enters through ostia into the spongocoel and out through the osculum.
• Choanocytes (collar cells) line the canals and help in food capture.
• Body supported by a skeleton of spicules or spongin fibres; mostly hermaphrodite.
• Examples: Sycon (Scypha), Spongilla, Euspongia.

Phylum Coelenterata (Cnidaria)

• Tissue level; diploblastic; radial symmetry.
• Possess stinging cnidoblasts (nematocysts) for defence and capture of prey.
• Central gastrovascular cavity with a single opening (hypostome-bearing mouth).
• Show two body forms — polyp (sessile) and medusa (free-swimming); alternation of generations (metagenesis) in some.
• Examples: Hydra, Adamsia (sea anemone), Physalia, Aurelia (jellyfish), Meandrina (coral).

Phylum Ctenophora (Comb Jellies / Sea Walnuts)

• Marine, diploblastic, radially symmetrical with tissue level of organisation.
• Eight rows of ciliated comb plates help in locomotion.
• Bioluminescence is well-marked; digestion is extracellular and intracellular.
• Examples: Pleurobrachia, Ctenoplana.

Phylum Platyhelminthes (Flatworms)

• Dorso-ventrally flattened body; bilaterally symmetrical; triploblastic; acoelomate.
• Mostly endoparasites; flame cells help in osmoregulation and excretion.
• Examples: Taenia (tapeworm), Fasciola (liver fluke), Planaria.

Phylum Aschelminthes (Roundworms)

• Circular body in cross-section; bilateral, triploblastic and pseudocoelomate.
• Sexes separate (dioecious); often sexual dimorphism is marked.
• Examples: Ascaris (roundworm), Wuchereria (filaria worm), Ancylostoma (hookworm).

Phylum Annelida (Segmented Worms)

• Bilateral, triploblastic, coelomate; body shows metameric segmentation.
• Possess closed circulatory system; nephridia for excretion; aquatic forms have parapodia.
• Examples: Pheretima (earthworm), Hirudinaria (leech), Nereis.

Phylum Arthropoda (Largest Phylum)

• Largest animal phylum; chitinous exoskeleton; jointed appendages.
• Open circulatory system; respiration by gills, book gills, book lungs or tracheal system; excretion by Malpighian tubules.
• Examples: Apis (honeybee), Bombyx (silkworm), Anopheles (mosquito), Limulus (king crab — a living fossil).

Phylum Mollusca (Second Largest Phylum)

• Soft body usually covered by a calcareous shell; body has head, muscular foot and visceral hump.
• Feeding organ is a rasping radula; open circulation in most.
• Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia, Octopus, Loligo.

Phylum Echinodermata (Spiny-skinned Animals)

• Exclusively marine; endoskeleton of calcareous ossicles (hence spiny).
• Adults show radial symmetry, larvae are bilaterally symmetrical.
• Unique water vascular system aids locomotion, capture and transport of food.
• Examples: Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria.

NEET trap: Echinoderms are the only non-chordate deuterostomes and share the deuterostome line with chordates.

Phylum Chordata — Protochordata to Mammalia

Fundamental Chordate Characters

All chordates possess, at some stage of life:

1. A notochord (dorsal supporting rod).
2. A dorsal hollow nerve cord.
3. Paired pharyngeal gill slits.
4. A post-anal tail and a closed circulatory system with a ventral heart.

NEET trap: In non-chordates the nerve cord is ventral, solid and double, whereas in chordates it is dorsal, hollow and single. The heart is dorsal in non-chordates but ventral in chordates.

Subphyla of Chordata

Urochordata and Cephalochordata are together called Protochordata (Acrania — lacking a skull).

Vertebrata — Living Chordates with a Backbone

Class Cyclostomata (Jawless Fishes)

• Jawless, eel-like ectoparasites; circular sucking mouth; 6–15 pairs of gill slits.
• Examples: Petromyzon (lamprey), Myxine (hagfish).

Class Chondrichthyes (Cartilaginous Fishes)

• Marine; cartilaginous endoskeleton; ventral mouth; placoid scales; no air bladder, so must swim constantly.
• Examples: Scoliodon (dogfish), Pristis (sawfish), Torpedo (electric ray).

Class Osteichthyes (Bony Fishes)

• Skeleton bony; air bladder present (regulates buoyancy); cycloid/ctenoid scales.
• Examples: Labeo (rohu), Hippocampus (sea horse), Catla, Exocoetus (flying fish).

Class Amphibia

• Can live on land and in water; two pairs of limbs; skin moist (no scales); three-chambered heart.
• Examples: Rana (frog), Bufo (toad), Salamandra, Ichthyophis (limbless amphibian).

Class Reptilia

• Creeping/crawling; body covered by dry, cornified skin with epidermal scales; three-chambered heart (crocodiles four).
• Examples: Chelone (turtle), Naja (cobra), Calotes (garden lizard), Crocodilus.

Class Aves (Birds)

• Presence of feathers; forelimbs modified into wings; bones are pneumatic (hollow, air-filled); four-chambered heart; oviparous.
• Examples: Corvus (crow), Columba (pigeon), Struthio (ostrich), Pavo (peacock).

Class Mammalia

• Presence of mammary glands for nourishing young; body covered with hair; external ears (pinnae); four-chambered heart; mostly viviparous.
• Examples: Ornithorhynchus (egg-laying platypus), Macropus (kangaroo — pouched), Homo sapiens (humans), Balaenoptera (blue whale).

Comparative Summary of Vertebrate Classes

NEET trap: Crocodiles are reptiles but have a four-chambered heart; whales, dolphins and bats are mammals, not fish or birds.

Frequently asked questions

Are these Animal Kingdom notes free?

Yes — the Animal Kingdom notes for Biology (Class 11) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Animal Kingdom notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Animal Kingdom chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Animal Kingdom.