6. Animal Classification

In this chapter we explore how and why animals are classified — from Aristotle to modern criteria — and tour the entire Kingdom Animalia from sponges to humans.

Can you recall?

Which criteria are used for classification of organisms?
What are the five kingdoms? Which kingdoms include unicellular vs multicellular life?

Hint: Cell type • Body plan • Symmetry • Germ layers • Coelom • Segmentation • Evolution

History of Animal Classification

Aristotle (ancient Greece): first broad attempt — based on size, habits, habitat → called an artificial system.
Theophrastus, Pliny, John Ray, Linnaeus: refined artificial methods; Linnaeus popularized binomial names.
Natural systems: based on body organization, tissues, body cavity, symmetry, embryos, biochemistry.
Evolutionary systems (Dobzhansky, Mayr): relationships explained via descent and change.
Modern era: Whittaker’s 5-kingdoms; molecular insights and Woese’s domains shape today’s view.

Benefits of Classification

Makes the vast diversity learnable.
Studying few members helps understand the whole group.
Reveals evolutionary relationships and adaptations.
Aids correct identification and conservation planning.

Living Organisms (Quick Map)

Modern view places “protozoa” (traditional) within Protista, not Animalia.

Traditional (Conventional) Classification

A. Non-Chordates

No notochord; gill slits absent; ventral solid nerve cord (if present); heart dorsal (if present).
Includes ten phyla (traditional): Protozoaunicellular, Porifera, Cnidaria, Platyhelminthes, Aschelminthes (Nemathelminthes), Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata.

B. Chordates

Notochord present at least in one stage; pharyngeal gill slits/ lungs for respiration.
Dorsal hollow nerve cord; ventral heart.
Phylum Chordata → Subphyla: Urochordata, Cephalochordata, Vertebrata.

New (Method) System — Key Criteria

1) Grades of Organization

Protoplasmic (unicellular protists e.g. Amoeba, Paramecium).
Cellular grade — cells loose; no true tissues (e.g. Porifera).
Cell-tissue grade — tissues present (e.g. Cnidaria).
Tissue-organ grade — organs start (e.g. flatworms).
Organ-system grade — systems integrated (e.g. annelids → vertebrates).

2) Body Symmetry

Asymmetrical — no equal halves (many sponges).
Radial — many planes around axis (starfish, jellyfish).
Bilateral — one plane only (insects, fishes, humans).

3) Germ Layers

Diploblastic: ectoderm + endoderm (e.g. Cnidaria).
Triploblastic: + mesoderm (most others).

4) Body Cavity (Coelom)

Acoelomate: no coelom (flatworms).
Pseudocoelomate: false coelom (roundworms).
Eucoelomate: true coelom (annelids → chordates).

5) Segmentation

Body divided into repeated units (e.g. earthworm) vs. unsegmented.

Quick Matrix (Kingdom: Animalia)

Criterion	Examples
Cellular grade	Porifera
Cell-tissue	Cnidaria
Tissue-organ	Platyhelminthes
Organ-system	Annelida, Arthropoda, Mollusca, Echinodermata, Chordata
Radial symmetry	Echinodermata (adult), Cnidaria
Acoelomate / Pseudocoelomate / Eucoelomate	Flatworms / Roundworms / Annelids onward

Kingdom Animalia — Phyla (Non-Chordates)

Protozoa Traditional

Unicellular eukaryotes (e.g. Amoeba, Paramecium). Note: In modern schemes they are placed in Protista, not Animalia.

Porifera — Sponges

Aquatic (mostly marine); body full of pores: ostia (in), osculum (out).
Cellular grade, asymmetrical, sedentary; collar cells present.
Skeleton: spicules (CaCO₃/SiO₂) or spongin fibres.
Filter feeders; reproduce by budding/sexual; high regeneration.
Examples: Sycon, Euspongia (bath sponge), Euplectella.

Cnidaria / Coelenterata

Polyp (cylindrical) & Medusa (umbrella) forms; mostly marine.
Diploblastic, radial symmetry.
Tentacles with cnidoblasts (stinging cells) — capture & defense.
Examples: Hydra, Aurelia (jellyfish), Adamsia (sea anemone), Physalia, Corals.

Coral reefs = massive cnidarian colonies; “coral” gemstone and ayurvedic pravāl bhasma come from them.

Platyhelminthes — Flatworms

Triploblastic, acoelomate, bilateral; flat, leaf/strip-like.
Many endoparasites; hermaphrodite.
Examples: Planaria, Liver fluke, Tapeworm.

Aschelminthes (Nemathelminthes) — Roundworms

Thread-like/cylindrical; tough cuticle; pseudocoelomate; unsegmented.
Free-living or parasitic; sexes separate.
Examples: Ascaris, filarial worm, Loa loa.

Annelida — Segmented Worms

Metameric segmentation; eucoelomate, bilateral.
Setae/parapodia/suckers for locomotion; cuticle.
Free-living (marine/freshwater/terrestrial) or ectoparasitic.
Examples: Earthworm, Nereis, Leech.

Earthworm is the “farmer’s friend”.

Arthropoda — Jointed-Leg Animals

Largest phylum; found in all habitats.
Chitinous exoskeleton; segmented; paired jointed appendages; sexes separate.
Examples: Crab, Spider, Scorpion, Millipede, Centipede, Cockroach, Butterfly, Honey bee.

Mollusca — Soft-bodied

Soft non-segmented body: head, foot, visceral mass; eucoelomate.
Mantle secretes shell (external/internal/absent); sexes separate.
Examples: Bivalves, Snail, Octopus.

Octopus: highly intelligent; can change colour; multiple locomotions.

Echinodermata — Spiny-skinned

Marine only; calcareous spines/ossicles; tube feet for movement & feeding.
Larvae bilateral; adults radial; strong regeneration.
Examples: Starfish, Sea urchin, Brittle star, Sea cucumber.

Hemichordata — “Half-Chordates”

Marine, burrowers; body: proboscis, collar, trunk.
Notochord only in proboscis region; gill slits present; sexes separate or hermaphrodite.
Examples: Balanoglossus, Saccoglossus (a connecting link view in evolution).

Phylum Chordata

Defining features: notochord (at least in one stage), dorsal tubular nerve cord, pharyngeal gill slits (or lungs), ventral heart.

Urochordata

Marine; body covered with tunic.
Larva free-swimming; notochord in tail only.
Adult often sedentary; mostly hermaphrodite.
Examples: Herdmania, Doliolum, Oikopleura.

Cephalochordata

Small fish-like; marine.
Notochord along entire body length; large pharynx with gill slits.
Sexes separate.
Example: Amphioxus (Branchiostoma).

Vertebrata / Craniata

Notochord replaced by vertebral column.
Well-developed head; brain in cranium.
Endoskeleton cartilaginous or bony.
Classes: Cyclostomata, Pisces, Amphibia, Reptilia, Aves, Mammalia.

Classes of Vertebrates — Key Features & Examples

Cyclostomata

Jaw-less; suctorial mouth; soft skin; no paired fins.
Cartilaginous skeleton; many are ectoparasites.
Examples: Petromyzon, Myxine.

Pisces (Fishes)

Cold-blooded; aquatic (fresh/marine); spindle-shaped body; paired & unpaired fins.
Exoskeleton (scales); endoskeleton cartilaginous/bony; gill respiration.
Examples: Rohu, Pomfret, Sea horse, Shark, Rays.

Amphibia

Larvae aquatic; adults live on land & water; respire by gills/skin/lungs.
Two pairs of limbs; moist skin; tympanum present; no claws.
Examples: Frog, Toad, Salamander.

Reptilia

First true terrestrial vertebrates; cold-blooded; creeping movement common.
Dry scaly skin; digits with claws; external ear absent.
Examples: Tortoise, Lizard, Snake, Crocodile.

Aves (Birds)

Warm-blooded; adapted to flight; body spindle-shaped to reduce air resistance.
Forelimbs → wings; feathers; beak (no teeth).
Examples: Peacock, Parrot, Pigeon, Duck, Penguin.

Mammalia

Mammary glands; warm-blooded; body with hair/fur.
Digits with nails/claws/hooves; body: head-neck-trunk-tail.
Examples: Human, Elephant, Kangaroo, Dolphin, Bat.

Think & Discuss

Gharial & crocodile live on land and in water — yet are reptiles, not amphibians (scales, amniotic eggs, etc.).
Whales & walruses are aquatic but mammals (hair, lungs, mammary glands) — not fishes.

Always remember: Variety of animals thrive around us. While observing or collecting data for study, do not harm animals or their habitats.

Quick Glossary

Notochord: dorsal supporting rod separating nerve cord from body tissues.
Coelom: body cavity between body wall & gut — a key evolutionary feature.
Diploblastic / Triploblastic: two vs three embryonic germ layers.
Metamerism: repetition of body segments (e.g. annelids).
Homeotherm / Poikilotherm: warm-blooded vs cold-blooded.

Study Tip Make a 2-column revision sheet: Feature vs Phyla/Classes showing it (e.g., “tube feet → Echinodermata”, “cnidoblasts → Cnidaria”, “mantle → Mollusca”, “feathers → Aves”).

Chapter 6 — Animal Classification: Exercise Solutions

Always remember: Varieties of animals live around us. While studying or observing, be careful to not harm any animal or its habitat.

1) Identify me

I am diploblastic & acoelomate. Which phylum?
Answer: Phylum Cnidaria (Coelenterata) — diploblastic body (ectoderm & endoderm), no true coelom.
Radial symmetry + water vascular system; called a “fish” though I am not — who am I?
Answer: Starfish (Sea star; Asterias) — Phylum Echinodermata.
I live in human small intestine; thread-like body with pseudocoelom. Which phylum?
Answer: Aschelminthes (Nemathelminthes) — example: Ascaris (roundworm).
Multicellular but no tissues formed. Name my phylum.
Answer: Porifera — cellular grade of organization; no true tissues.
To which phylum does cockroach belong? Justify.
Answer: Arthropoda — segmented body (head–thorax–abdomen), jointed appendages, chitinous exoskeleton, open circulatory system, tracheal respiration.

2) Characters of given animals (using classification chart)

Animal	Group (Phylum / Class)	Key Characters
Bath sponge	Porifera	Ostia & osculum; collar cells; asymmetrical; spicules/spongin; filter feeder; sedentary.
Grasshopper	Arthropoda (Insecta)	3 body regions; 3 pairs of legs; wings; tracheal respiration; chitin exoskeleton; sexes separate.
Rohu	Chordata — Class Pisces (bony fish)	Streamlined; fins; scales; gill respiration; cold-blooded.
Penguin	Chordata — Class Aves	Warm-blooded; feathers; forelimbs → flipper-like wings; beak; lays eggs; flightless but bird.
Frog	Chordata — Class Amphibia	Moist skin; aquatic larvae & terrestrial adult; lung/skin respiration; no claws; cold-blooded.
Lizard (Wall lizard)	Chordata — Class Reptilia	Dry scaly skin; claws; lungs; amniotic eggs; cold-blooded.
Elephant	Chordata — Class Mammalia	Mammary glands; hair; warm-blooded; viviparous; diaphragm present.
Jellyfish	Cnidaria	Medusa form; radial symmetry; diploblastic; tentacles with cnidoblasts (stinging cells).

3) Progressive changes in animal classification (brief)

Aristotle: artificial grouping by size, habit, habitat.
Linnaeus & others: refined artificial methods; binomial nomenclature.
Natural systems: internal structure, tissues, coelom, development, biochemistry.
Evolutionary systems (Dobzhansky/Mayr): phylogeny & descent inform groupings.
Whittaker (5 Kingdoms): Monera, Protista, Fungi, Plantae, Animalia.
Modern updates: molecular data & Woese’s domains; within Animalia, criteria like grade of organization, symmetry, germ layers, coelom, segmentation + embryology.

4) Grades of organization vs Symmetry (with examples)

Aspect	Meaning	Examples
Grades of Organization	Levels of body construction: from loose cells to tissues to integrated organ systems.	Cellular: Porifera • Cell-tissue: Cnidaria • Tissue-organ: Platyhelminthes • Organ-system: Annelida → Vertebrata
Symmetry	How a body can be divided into equal halves by imaginary planes.	Asymmetry: sponges • Radial: starfish, jellyfish • Bilateral: earthworm, insects, fishes, humans

5) Answer in brief

a) Scientific classification of shark (up to class)

Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Pisces (as per textbook convention; sharks are cartilaginous fishes)

b) Four distinguishing characters of Echinodermata

Exclusively marine; adult radial symmetry (larva bilateral).
Calcareous endoskeleton of spines/ossicles.
Water vascular system with tube feet (locomotion/feeding).
High power of regeneration; sexes separate.

c) Distinguish: Butterfly vs Bat (any four points)

Feature	Butterfly	Bat
Group	Arthropoda (Insecta)	Mammalia
Skeleton	Exoskeleton (chitin)	Endoskeleton (bone)
Body covering	Scaled wings	Hair/fur; patagium-supported wings
Temperature	Cold-blooded	Warm-blooded
Respiration	Tracheal system	Lungs
Reproduction	Oviparous; metamorphosis	Viviparous; mammary glands

6) Give scientific reasons

Tortoise is not Amphibia though it lives on land & in water.
It has reptilian traits: dry scaly skin, lungs (no skin respiration), amniotic eggs, claws; hence Reptilia, not Amphibia.
Contact with jellyfish irritates skin.
Tentacles bear cnidoblasts (nematocysts) that inject toxins causing burning/irritation.
All vertebrates are chordates but all chordates are not vertebrates.
Vertebrates possess a vertebral column (a subset of Chordata). Urochordata & Cephalochordata lack vertebral column but are chordates.
Balanoglossus is a connecting link.
Shows chordate-like features (pharyngeal gill slits; proboscis notochord-like structure) and non-chordate traits; hence links non-chordates & chordates.
Reptile body temperature is not constant.
Reptiles are poikilothermic (ectothermic); they depend on environmental heat, so body temperature varies.

7) MCQs — choose the correct option

Special cells in sponges (Porifera): 1) Collar cells ✅
Bilateral symmetry shown by: 3) Earthworm ✅
Regenerates broken body part: 4) Star fish ✅
Bat is included in class: 4) Mammalia ✅

8) Complete the chart (Germ layers & Body cavity)

Phylum	Germ layers	Body cavity (Coelom)
Porifera	No true germ layers (cellular grade)	Absent
Cnidaria	Diploblastic	Absent (acoelomate)
Platyhelminthes	Triploblastic	Absent (acoelomate)
Aschelminthes	Triploblastic	Pseudocoelom present
Arthropoda	Triploblastic	Eucoelom present (reduced to hemocoel)

9) Complete the chart (Types • Character • Example)

Type (Class)	One Key Character	Example
Cyclostomata	Jawless; suctorial mouth; gill respiration	Petromyzon (lamprey)
Pisces	Gills; fins; poikilothermic	Rohu, Shark
Amphibia	Larva aquatic; adult on land & water; poikilothermic	Frog
Reptilia	Dry scales; lungs; poikilothermic	Lizard, Tortoise
Aves	Feathers; wings; warm-blooded	Pigeon, Penguin
Mammalia	Mammary glands; hair; warm-blooded	Elephant, Whale

Note: Whale is a mammal, not a fish.

10) Sketch, label & classify (guidance)

Organism	Classification	Key labels to include in your sketch
Hydra	Cnidaria	Tentacles, mouth, gastrovascular cavity, basal disc (polyp form)
Jellyfish	Cnidaria	Bell (umbrella), tentacles, oral arms, gonads (medusa)
Planaria	Platyhelminthes	Eyespots, pharynx, branched gut, flattened body
Roundworm	Aschelminthes	Cylindrical body, mouth, anus, simple gut, smooth cuticle
Butterfly	Arthropoda (Insecta)	Head–thorax–abdomen, antennae, wings with scales, 3 pairs of legs
Earthworm	Annelida	Segments, clitellum, anterior/posterior ends, setae (if shown)
Octopus	Mollusca	Head, eyes, arms with suckers, siphon
Star fish	Echinodermata	Five arms, tube feet (underside), madreporite
Shark	Chordata — Pisces	Dorsal/pectoral/pelvic/caudal fins, gill slits, lateral line
Frog	Chordata — Amphibia	Tympanum, forelimbs/hindlimbs, webbed feet, nares
Wall lizard	Chordata — Reptilia	Scaly skin, claws, tail, limbs
Pigeon	Chordata — Aves	Beak, wings, feathers, tail, claws

11) Label the following (recap from Q8)

Body cavity: Absent → Porifera, Cnidaria, Platyhelminthes • Pseudocoelom present: Aschelminthes • Eucoelom present: Arthropoda (and higher).
Germ layers: Diploblastic → Cnidaria • Triploblastic → Platyhelminthes onward.