Diversity in the Animal Kingdom ICSE Class 9: Non-chordates from Porifera to Echinodermata and the Five Chordate Classes

What Diversity in the Animal Kingdom is about

Weightage: CISCE publishes no chapter weightage for ICSE Biology. Diversity in the Animal Kingdom is topic 4, Diversity in Animal Kingdom, of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2028 (Class IX)

Key concepts

Chordates and non-chordates

Chordates (phylum Chordata) have, at some stage of life:
1. a notochord: a flexible rod along the back that supports the body (replaced by the backbone in vertebrates);
2. a dorsal, hollow nerve cord (spinal cord);
3. pharyngeal gill slits (in the throat region; in land vertebrates only in the embryo);
4. a post-anal tail (at least in the embryo).

Non-chordates (invertebrates) lack a notochord; their nerve cord, if present, is ventral and solid; they have no gill slits in the pharynx.

Feature	Non-chordates	Chordates
Notochord	absent	present (at some stage)
Nerve cord	ventral, solid (often double)	dorsal, hollow, single
Pharyngeal gill slits	absent	present at some stage
Heart (if present)	dorsal	ventral

Worked example

A human embryo has gill slits and a tail, which disappear before birth. Why are humans still called chordates?

Try it first, then show the solutionSolution

1. Chordate features need be present only at some stage of life.
2. Humans have a notochord, gill slits and a tail in the embryo, and a dorsal hollow nerve cord throughout.

Answer: They show the chordate features in the embryo.

Porifera and Cnidaria

Phylum Porifera (sponges): 'pore-bearers'.

• Simplest multicellular animals: cellular level of organisation, no true tissues or organs.
• Body full of pores (ostia); water enters through them, flows through a canal system and leaves by a large opening, the osculum, bringing food and oxygen.
• Sessile (fixed), mostly marine; skeleton of spicules or spongy fibres.
• Examples: Sycon, bath sponge (Euspongia), Spongilla (freshwater).

Phylum Cnidaria (Coelenterata):

• Tissue level of organisation; body wall of two layers (diploblastic).
• Radial symmetry.
• A single central cavity, the coelenteron (gastrovascular cavity), with one opening (mouth) used for both taking in food and removing waste.
• Tentacles around the mouth bear stinging cells (cnidoblasts) for catching prey and defence.
• Examples: Hydra, jellyfish (Aurelia), sea anemone, corals.

Worked example

Why can a sponge live fixed to a rock and still get food?

Try it first, then show the solutionSolution

1. Water flows in through its pores and out through the osculum.
2. The current carries tiny food particles and oxygen past its cells.

Answer: Its canal system brings food in the water current.

Platyhelminthes, Nematoda and Annelida (the worms)

Phylum Platyhelminthes (flatworms):

• Body flattened dorsoventrally, unsegmented (or with ribbon-like units in tapeworms), bilateral symmetry, three germ layers, no body cavity (acoelomate).
• Many are parasites; hooks and suckers for attachment; excretion by flame cells; mostly hermaphrodite.
• Examples: tapeworm (Taenia), liver fluke (Fasciola), Planaria (free-living).

Phylum Nematoda (roundworms):

• Body long, cylindrical, unsegmented, tapering at both ends, with a tough cuticle.
• A false body cavity (pseudocoelom); complete gut (mouth and anus); sexes separate.
• Many are parasites.
• Examples: Ascaris (roundworm of the intestine), filarial worm (Wuchereria), hookworm.

Phylum Annelida (segmented worms):

• Body divided into ring-like segments (metameric segmentation) inside and out.
• True body cavity (coelom); closed blood system.
• Move by setae (bristles) or parapodia; breathe through moist skin.
• Examples: earthworm (Pheretima), Nereis (sandworm), leech.

Worked example

How would you tell an earthworm from a roundworm such as Ascaris by looking at the body?

Try it first, then show the solutionSolution

1. The earthworm's body is divided into many ring-like segments.
2. Ascaris is smooth, unsegmented and tapers at both ends.

Answer: Segmented (annelid) versus unsegmented (nematode).

Arthropoda, Mollusca and Echinodermata

Phylum Arthropoda ('jointed feet'): the largest phylum.

• Jointed legs (appendages).
• Hard exoskeleton of chitin, shed during growth (moulting).
• Segmented body, usually in head, thorax and abdomen (or cephalothorax and abdomen).
• Open blood system (blood in a body space, the haemocoel).
• Examples: insects (cockroach, butterfly, housefly), crustaceans (prawn, crab), arachnids (spider, scorpion), centipedes and millipedes.

Phylum Mollusca ('soft-bodied'):

• Soft, unsegmented body, usually with a head, a muscular foot and a visceral mass.
• A fold of skin, the mantle, which usually secretes a hard calcareous shell.
• Breathe by gills.
• Examples: snail (Pila), mussel, oyster, octopus, squid (shell reduced or absent).

Phylum Echinodermata ('spiny skin'):

• Exclusively marine.
• Spiny skin with an internal skeleton of calcareous plates.
• Adults show radial (five-part) symmetry; larvae are bilateral.
• A unique water vascular system working tube feet for movement, feeding and breathing.
• Examples: starfish, sea urchin, sea cucumber, brittle star.

Worked example

Why must a crab or prawn moult as it grows?

Try it first, then show the solutionSolution

1. It has a hard chitinous exoskeleton that cannot stretch.
2. To grow, it sheds the old skeleton and forms a larger one.

Answer: The rigid exoskeleton limits growth.

Fishes (Pisces) and amphibians (Amphibia)

Vertebrate chordates have a backbone (vertebral column) that replaces the notochord, a skull, and paired limbs or fins.

Class Pisces (fishes):

• Fully aquatic; streamlined body.
• Skin covered with scales; fins for swimming and balance; gills for breathing (often covered by an operculum in bony fish).
• Two-chambered heart; cold-blooded (body temperature changes with surroundings).
• Examples: rohu (Labeo), shark (cartilaginous skeleton), seahorse.

Class Amphibia ('double life'):

• Live both in water and on land; must return to water to breed.
• Moist, glandular skin without scales; breathe by lungs and skin (larvae, called tadpoles, by gills).
• Three-chambered heart; cold-blooded.
• Eggs laid in water; external fertilisation; larva undergoes metamorphosis.
• Examples: frog, toad, salamander.

Worked example

Why does a frog's skin need to stay moist?

Try it first, then show the solutionSolution

1. A frog breathes partly through its skin.
2. Oxygen dissolves in the moist surface before diffusing into the blood.

Answer: For gas exchange through the skin.

Reptiles, birds and mammals

Class Reptilia:

• Truly land animals (some return to water).
• Dry skin with horny scales; breathe by lungs only.
• Heart usually three-chambered (four in crocodiles); cold-blooded.
• Lay eggs with leathery shells on land; internal fertilisation.
• Examples: lizard, snake, turtle, crocodile.

Class Aves (birds):

• Body covered with feathers; forelimbs modified as wings; beak without teeth.
• Hollow (pneumatic) bones and air sacs make the body light for flight.
• Four-chambered heart; warm-blooded (constant body temperature).
• Lay hard-shelled eggs; incubate them.
• Examples: pigeon, sparrow, ostrich (flightless), penguin.

Class Mammalia:

• Body covered with hair; mammary glands feed the young with milk.
• External ears (pinnae); a muscular diaphragm separates chest and abdomen.
• Four-chambered heart; warm-blooded.
• Most give birth to live young (viviparous); a few lay eggs (duck-billed platypus).
• Examples: human, cow, bat (flying), whale and dolphin (aquatic).

Worked example

A bat flies and a whale lives in the sea. Why are both mammals and not a bird and a fish?

Try it first, then show the solutionSolution

1. Both have hair (whales have a few bristles), give birth to live young and feed them milk from mammary glands.
2. Both breathe air with lungs and are warm-blooded with a four-chambered heart.

Answer: They have the defining mammal features.

Diversity in the Animal Kingdom in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Chordates: Fishes, Amphibians, Reptiles, Birds and Mammals
• Non-Chordate Phyla: Porifera to Echinodermata

Diversity in the Animal Kingdom: key facts

Non-chordate phyla in order

$\text{Porifera}\to \text{Cnidaria}\to \text{Platyhelminthes}\to \text{Nematoda}\to \text{Annelida}\to \text{Arthropoda}\to \text{Mollusca}\to \text{Echinodermata}$

Increasing complexity.

Chordate classes

$\text{Pisces}\to \text{Amphibia}\to \text{Reptilia}\to \text{Aves}\to \text{Mammalia}$

Five classes.

Heart chambers

$\text{fish }2;\text{amphibian }3;\text{reptile }3(\text{crocodile }4);\text{bird }4;\text{mammal }4$

Warm-blooded: birds, mammals.

Chordate features

$\text{notochord}+\text{dorsal hollow nerve cord}+\text{gill slits}+\text{post-anal tail}$

At some stage of life.

Diversity in the Animal Kingdom: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Diversity in the Animal Kingdom

The question patterns to expect, and the method that works for each.

Tricks to remember Diversity in the Animal Kingdom

Short memory aids. Say each one aloud once, then use it on a question the same day.

• 'Please Come, Players Need A Ball, My Equipment'

  Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata.

  Use it when: Order of non-chordate phyla.

• 'Fish Are Really Awesome Mates'

  Pisces, Amphibia, Reptilia, Aves, Mammalia.

  Use it when: Five chordate classes.

• Name tells the feature

  Porifera = pores; Cnidaria = stinging cells; Platy = flat; Nema = thread; Annelida = rings; Arthro = jointed; Mollusca = soft; Echino = spiny.

  Use it when: Defining features.

• Starfish, jellyfish, silverfish: no fish

  Starfish is an echinoderm, jellyfish a cnidarian, silverfish an insect.

  Use it when: Common traps.

Diversity in the Animal Kingdom: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Diversity in the Animal Kingdom

The slips examiners see most often, and what to write instead.

• ✗Mistake: Classifying by name or habitat

  Starfish, jellyfish and silverfish are taken as fish; whales as fish because they swim.

  Wrong working

  Starfish: Pisces. Whale: Pisces.

  Right working

  Starfish: Echinodermata. Whale: Mammalia.

  ✓Do this: Use body features, not names or habitats.

  Usually costs: 1 mark each.

• ✗Mistake: Nematoda and Annelida mixed

  Both are 'worms'.

  Wrong working

  Ascaris is an annelid.

  Right working

  Ascaris: Nematoda (unsegmented); earthworm: Annelida (segmented).

  ✓Do this: Segments → Annelida.

  Usually costs: 1 mark.

• ✗Mistake: Heart chambers wrong

  All vertebrates assumed to have four chambers.

  Wrong working

  Frog has a four-chambered heart.

  Right working

  Fish 2, amphibian 3, reptile 3 (crocodile 4), bird and mammal 4.

  ✓Do this: Learn the 2-3-3-4-4 sequence.

  Usually costs: 1 mark.

• ✗Mistake: Feature written without the group or example

  Answers list features but do not say which group, or give wrong examples.

  Wrong working

  Jointed legs, exoskeleton: earthworm.

  Right working

  Arthropoda: jointed legs, chitinous exoskeleton; e.g. cockroach.

  ✓Do this: Group + features + correct example.

  Usually costs: 1 mark.

Practice questions on Diversity in the Animal Kingdom

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which animal belongs to Mollusca?
   (a) starfish
   (b) octopus
   (c) jellyfish
   (d) prawn

   Hint

   Soft body, mantle.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   A three-chambered heart and moist skin without scales are features of:
   (a) fishes
   (b) amphibians
   (c) reptiles
   (d) birds

   Hint

   Frog.

   Answer

   (b)

3. Q3 · 2 marks · Very short answerExam level

   Name the class: (a) penguin, (b) turtle, (c) seahorse, (d) dolphin.

   Hint

   Not by habitat.

   Answer

   (a) Aves (b) Reptilia (c) Pisces (d) Mammalia

4. Q4 · 2 marks · Short answerExam level

   Give two features of Porifera and two examples.

   Hint

   Pores.

   Answer

   Body with many pores and a canal system with an osculum; sessile, mostly marine with spicules. Examples: Sycon, bath sponge (Euspongia), Spongilla.

5. Q5 · 2 marks · Short answerHarder exam level

   Give two differences between Platyhelminthes and Nematoda.

   Hint

   Shape; cavity.

   Answer

   Platyhelminthes: flattened body, no body cavity (acoelomate), usually hermaphrodite. Nematoda: cylindrical body, pseudocoelom, sexes separate (complete gut).

6. Q6 · 2 marks · Very short answerHarder exam level

   Name the phylum in which each feature is found: (a) metameric segmentation, (b) mantle, (c) chitinous exoskeleton, (d) coelenteron.

   Hint

   One phylum each.

   Answer

   (a) Annelida (b) Mollusca (c) Arthropoda (d) Cnidaria

7. Q7 · 3 marks · Short answerHarder exam level

   How are birds adapted for flight? Give three features.

   Hint

   Light and strong.

   Answer

   Forelimbs modified into wings with feathers; hollow (pneumatic) bones and air sacs make the body light; streamlined body (also a strong breastbone for flight muscles).

8. Q8 · 2 marks · Short answerHarder exam level

   Why is a crocodile placed with reptiles even though it lives in water and has a four-chambered heart?

   Hint

   Scales, lungs, eggs.

   Answer

   It has dry skin with horny scales, breathes only by lungs, is cold-blooded and lays shelled eggs on land: reptile features. The four-chambered heart is an exception within the class.

9. Q9 · 3 marks · Short answerHarder exam level

   Give three differences between chordates and non-chordates.

   Hint

   Notochord, nerve cord, gill slits.

   Answer

   Notochord: present versus absent. Nerve cord: dorsal, hollow versus ventral, solid. Pharyngeal gill slits: present at some stage versus absent.

10. Q10 · 3 marks · Short answerHOTS / competency

    An animal has a soft body with no shell, eight arms with suckers, lives in the sea and has a mantle. Another lives in the sea, has five arms, spiny skin and moves by tube feet. Identify the phylum of each and say why the first is not an echinoderm.

    Hint

    Octopus and starfish.

    Answer

    First: Mollusca (octopus): soft body and mantle. Second: Echinodermata (starfish). The octopus has no spiny skin, no calcareous plates and no water vascular system or tube feet, so it is not an echinoderm.

11. Q11 · 3 marks · Short answerHOTS / competency

    A student says, 'All animals that live in water breathe with gills.' Disprove this with one example each from three different chordate classes.

    Hint

    Air-breathers in water.

    Answer

    Whale or dolphin (Mammalia) breathes air with lungs; turtle or crocodile (Reptilia) breathes with lungs; adult frog (Amphibia) breathes with lungs and skin; penguin (Aves) uses lungs.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Largest animal phylum?Show answer

   Arthropoda.

2. 2.Phylum with stinging cells?Show answer

   Cnidaria.

3. 3.Excretory cells of flatworms?Show answer

   Flame cells.

4. 4.Phylum of the earthworm?Show answer

   Annelida.

5. 5.Heart chambers of a fish?Show answer

   Two.

6. 6.Egg-laying mammal?Show answer

   Duck-billed platypus.

7. 7.Class of animals with feathers?Show answer

   Aves.

8. 8.Rod-like support along the back of chordates?Show answer

   Notochord.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you list the eight non-chordate phyla in order?
• ☐Can you give one defining feature for each phylum?
• ☐Can you give two examples for each phylum and class?
• ☐Can you state the four chordate features?
• ☐Can you compare fishes, amphibians, reptiles, birds and mammals in a table?

Diversity in the Animal Kingdom: frequently asked questions