CDS Biology · Biodiversity and Classification
Animal Phyla and Classes
The animal phyla from sponges to echinoderms, the organs animals and protists move with, and the chordate classes with their hearts and body temperature.
Why this matters
Eleven CDS questions from 2018 (I) to 2025 (I), three of them in the 2018 (I) paper. Five are about chordates, and the three-chambered heart has been asked twice. Nereis, the sea worm, has also come up twice: once for its symmetry and once for how it moves.
Concept 1 of 4: Sponges, cnidarians and worms
The simplest animals are sorted by their body plan. Sponges have no real tissues, only a body full of pores. Cnidarians such as Hydra are built round a centre, like a wheel, so any cut through the middle gives two equal halves. Worms have a head end and a tail end, so only one cut, down the middle, gives two mirror halves: that is bilateral symmetry.
Definition
The non-chordate phyla up to the annelids (NCERT Class 9 and 11):
- Porifera: the sponges; a body full of pores through which water flows; no true tissues; mostly marine; for example Sycon, Spongilla.
- Cnidaria (Coelenterata): radial symmetry; stinging cells (cnidoblasts); for example Hydra, jellyfish, sea anemone.
- Platyhelminthes: flatworms; bilateral symmetry; mostly parasites (liver fluke, tapeworm) but Planaria is free-living in fresh water.
- Aschelminthes (Nematoda): roundworms; for example Ascaris and Wuchereria (filarial worm), both parasites.
- Annelida: segmented worms with bilateral symmetry; for example earthworm, leech and Nereis, a marine worm.
- From the flatworms upward, nearly all animals are bilaterally symmetrical. The echinoderms are the odd case: the adults are radial, the larvae bilateral.
| Phylum | Body plan | Examples |
|---|---|---|
| Porifera | Pores, no true tissues | Sycon, Spongilla |
| Cnidaria | Radial symmetry, stinging cells | Hydra, jellyfish, sea anemone |
| Platyhelminthes | Flat, bilateral symmetry | Planaria (free-living), liver fluke, tapewormQ CDS 2021 (I): Planaria is the free-living animal; liver fluke, Wuchereria and Plasmodium are parasites. |
| Aschelminthes | Round, unsegmented | Ascaris, Wuchereria |
| Annelida | Segmented, bilateral symmetry | Earthworm, leech, NereisQ CDS 2019 (I): Nereis is the bilaterally symmetrical one; sea anemone, starfish and sea urchin are not. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q76Easy
Example 1 · Biodiversity and Classification · Animal Phyla and Classes
Sponges belong to which one among the following phylum ?
Not every flatworm is a parasite
Liver fluke and tapeworm are parasites, but Planaria is a free-living flatworm of fresh water. Plasmodium, often listed beside them, is not a worm at all: it is a single-celled protist.
Starfish are radial as adults
Starfish (Asterias) and sea urchins (Echinus) are echinoderms whose adults have five-part radial symmetry. The bilaterally symmetrical choice is the worm, such as Nereis.
Concept 2 of 4: Organs of locomotion
Each group of small animals and protists moves with its own tool. Some lash a long whip, some beat many tiny hairs, and Amoeba simply pushes out a bulge of its body and flows into it. Learn each pair, because the papers mix them up and ask which pair is wrong.
Definition
How some organisms move (NCERT Class 11):
- Amoeba: pseudopodia (false feet), temporary outgrowths of the cell.
- Euglena: a long flagellum, a whip (it has a second, very short one).
- Paramecium: cilia, many short hairs that beat together.
- Nereis: parapodia, flat side flaps on each segment, used for swimming.
- Earthworm: muscles in the body wall, helped by tiny bristles called setae.
- Starfish: tube feet, worked by the water vascular system.
- Snail and other molluscs: a muscular foot.
| Organism | Organ of locomotion |
|---|---|
| Amoeba | Pseudopodia |
| Euglena | Flagellum |
| Paramecium | Cilia |
| Nereis | ParapodiaQ CDS 2018 (I): Nereis with pseudopodia is the wrongly matched pair. |
| Earthworm | Body-wall muscles and setae |
| Starfish | Tube feet |
| Snail | Muscular foot |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2018 · CDS (I) 2018 — General Knowledge · Q21Moderate
Example 2 · Biodiversity and Classification · Animal Phyla and Classes
Which one of the following pairs about organ/part that helps in locomotion is not correctly matched?
Pseudopodia belong to Amoeba
Pseudopodia are the temporary bulges of Amoeba. The sea worm Nereis swims with parapodia, flaps on its segments. The two words differ by two letters, which is the whole trap.
Concept 3 of 4: Arthropods, molluscs and echinoderms
Arthropods are the largest phylum on Earth: insects, spiders, crabs and centipedes. Their one sure sign is jointed legs. Molluscs have a soft body, often inside a shell, and creep on a muscular foot. Echinoderms are sea animals with spiny skin, and they move on tube feet.
Definition
Three big non-chordate phyla:
- Arthropoda: jointed legs (the name means jointed feet), a hard exoskeleton of chitin, a segmented body. Insects have six legs and three body parts; spiders have eight legs.
- Mollusca: soft, unsegmented body, usually with a shell, and a muscular foot; for example Pila (apple snail), oyster, octopus, cuttlefish. Pearls form inside the pearl oyster, which lays down layers of nacre around a grain that gets inside its shell.
- Echinodermata: spiny skin, a water vascular system with tube feet; all marine; for example starfish (Asterias), sea urchin (Echinus).
- Segmentation, a long body and bilateral symmetry are found in both annelids and arthropods. Only jointed legs tell an insect from an earthworm.
| Phylum | Sure sign | Examples |
|---|---|---|
| Arthropoda | Jointed legs, exoskeleton | Insects, spiders, prawn, silverfishQ CDS 2021 (II): jointed legs confirm that an animal is an insect and not an earthworm. |
| Mollusca | Soft body, shell, muscular foot | Oyster, Pila, octopus, cuttlefishQ CDS 2020 (II): pearls are harvested from oysters. |
| Echinodermata | Spiny skin, tube feet | Starfish, sea urchin |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2021 · CDS (II) 2021 — General Knowledge · Q28Easy
Example 3 · Biodiversity and Classification · Animal Phyla and Classes
Rupa and Sachin observed an animal in their school garden. Rupa called it an insect while Sachin identified it as an earthworm. Which one of the following characteristics confirms that it is an insect ?
Segments do not make an insect
An earthworm is segmented, long and bilaterally symmetrical too. The feature that only an arthropod has is jointed legs.
Pearls come from oysters, not snails
Pila (the apple snail) is also a mollusc, but pearls are made by the pearl oyster. Prawn is an arthropod and tuna a fish.
Concept 4 of 4: Chordates and their hearts
Every chordate has a notochord, a stiff rod along its back, at some stage of life. In the vertebrates a backbone grows in its place. The vertebrate classes can then be told apart by their hearts: two chambers in fish, three in amphibians and most reptiles, four in birds and mammals. A four-chambered heart keeps clean and used blood apart, which lets birds and mammals keep a steady body heat.
Definition
Phylum Chordata (NCERT Class 9):
- Protochordata: marine, bilaterally symmetrical, with a coelom and a notochord, but the notochord is never replaced by a vertebral column; for example Balanoglossus, Herdmania, Amphioxus.
- Vertebrata: the notochord is replaced by a backbone. Five classes:
- Pisces (fish): gills, fins, two-chambered heart, cold-blooded.
- Amphibia (frog, toad, salamander): moist skin, three-chambered heart, cold-blooded.
- Reptilia (lizard, snake, tortoise, crocodile): dry scaly skin; most have a three-chambered heart, but crocodiles have four; cold-blooded.
- Aves (birds) and Mammalia (mammals): four-chambered heart and warm-blooded (body temperature stays steady).
| Group | Heart and body temperature | Examples |
|---|---|---|
| Protochordata | Notochord, but never a backbone; cold-blooded | Balanoglossus, Herdmania, AmphioxusQ CDS 2021 (II): marine, with a notochord, but never a vertebral column: Protochordata. |
| Pisces | Two chambers; cold-blooded | Rohu, shark (Scoliodon), flying fishQ CDS 2018 (I): of silverfish, jellyfish, cuttlefish and flying fish, only the flying fish is a true fish. |
| Amphibia | Three chambers; cold-blooded | Frog, toad, salamanderQ CDS 2018 (I): the classes with a three-chambered heart are Amphibia and Reptilia. |
| Reptilia | Three chambers (four in crocodiles); cold-blooded | Lizard, snake, tortoise, crocodile |
| Aves | Four chambers; warm-blooded | Pigeon, peacock, ostrichQ CDS 2019 (I): the warm-blooded pair is peacock and camel. |
| Mammalia | Four chambers; warm-blooded | Camel, bat, whale, human being |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q15Moderate
Example 4 · Biodiversity and Classification · Animal Phyla and Classes
Which one of the following animals has a three-chambered heart?
Only birds and mammals are warm-blooded
Crocodiles, tortoises and all fish are cold-blooded, even large ones. A warm-blooded pair must be a bird and a mammal, such as peacock and camel.
Most fish names are not fish
Silverfish is an insect, jellyfish a cnidarian, cuttlefish and starfish are not fish either (a mollusc and an echinoderm). A true fish has gills, fins and a backbone, like the flying fish.
Reptiles are three, except the crocodile
Amphibians and most reptiles have a three-chambered heart. So 'Reptilia only' or 'Amphibia only' is too narrow when both classes are offered together. The crocodile, a reptile, has four.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
Sponges, cnidarians and worms5 rows
| Phylum | Body plan | Examples |
|---|---|---|
| Porifera | Pores, no true tissues | Sycon, Spongilla |
| Cnidaria | Radial symmetry, stinging cells | Hydra, jellyfish, sea anemone |
| Platyhelminthes | Flat, bilateral symmetry | Planaria (free-living), liver fluke, tapewormQ CDS 2021 (I): Planaria is the free-living animal; liver fluke, Wuchereria and Plasmodium are parasites. |
| Aschelminthes | Round, unsegmented | Ascaris, Wuchereria |
| Annelida | Segmented, bilateral symmetry | Earthworm, leech, NereisQ CDS 2019 (I): Nereis is the bilaterally symmetrical one; sea anemone, starfish and sea urchin are not. |
Organs of locomotion7 rows
| Organism | Organ of locomotion |
|---|---|
| Amoeba | Pseudopodia |
| Euglena | Flagellum |
| Paramecium | Cilia |
| Nereis | ParapodiaQ CDS 2018 (I): Nereis with pseudopodia is the wrongly matched pair. |
| Earthworm | Body-wall muscles and setae |
| Starfish | Tube feet |
| Snail | Muscular foot |
Arthropods, molluscs and echinoderms3 rows
| Phylum | Sure sign | Examples |
|---|---|---|
| Arthropoda | Jointed legs, exoskeleton | Insects, spiders, prawn, silverfishQ CDS 2021 (II): jointed legs confirm that an animal is an insect and not an earthworm. |
| Mollusca | Soft body, shell, muscular foot | Oyster, Pila, octopus, cuttlefishQ CDS 2020 (II): pearls are harvested from oysters. |
| Echinodermata | Spiny skin, tube feet | Starfish, sea urchin |
Chordates and their hearts6 rows
| Group | Heart and body temperature | Examples |
|---|---|---|
| Protochordata | Notochord, but never a backbone; cold-blooded | Balanoglossus, Herdmania, AmphioxusQ CDS 2021 (II): marine, with a notochord, but never a vertebral column: Protochordata. |
| Pisces | Two chambers; cold-blooded | Rohu, shark (Scoliodon), flying fishQ CDS 2018 (I): of silverfish, jellyfish, cuttlefish and flying fish, only the flying fish is a true fish. |
| Amphibia | Three chambers; cold-blooded | Frog, toad, salamanderQ CDS 2018 (I): the classes with a three-chambered heart are Amphibia and Reptilia. |
| Reptilia | Three chambers (four in crocodiles); cold-blooded | Lizard, snake, tortoise, crocodile |
| Aves | Four chambers; warm-blooded | Pigeon, peacock, ostrichQ CDS 2019 (I): the warm-blooded pair is peacock and camel. |
| Mammalia | Four chambers; warm-blooded | Camel, bat, whale, human being |
Watch out for (8)
- Not every flatworm is a parasite→ Sponges, cnidarians and worms
- Starfish are radial as adults→ Sponges, cnidarians and worms
- Pseudopodia belong to Amoeba→ Organs of locomotion
- Segments do not make an insect→ Arthropods, molluscs and echinoderms
- Pearls come from oysters, not snails→ Arthropods, molluscs and echinoderms
- Only birds and mammals are warm-blooded→ Chordates and their hearts
- Most fish names are not fish→ Chordates and their hearts
- Reptiles are three, except the crocodile→ Chordates and their hearts
Test yourself on Biodiversity and Classification
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.