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Animals cannot make their own food, so they move, sense and respond — searching for food, escaping predators and interacting with their world.
- Animals are multicellular heterotrophs that depend on other organisms for food.
- Most show locomotion , rapid response to stimuli and coordinated behaviour.
- A major criterion for classifying animals is the presence or absence of a notochord (a flexible rod-shaped structure).
NCERT Question 2 — Sponges represent one of the
NCERT Question 15 — During a long-term ecological study,
- Based on the notochord, animals are split into non-chordata (invertebrata) and chordata .
- Invertebrates lack a notochord; chordates have one, in which the notochord can act as a precursor to the vertebral column.
- Chordata is further classified into protochordata and vertebrata .
Porifera (pore-bearers) — Multicellularity without tissues
Sponges are multicellular but lack tissues and organs. Water flows continuously through the pores of the body, bringing food particles and oxygen to individual cells and carrying waste away.
Cnidaria — True tissues and active feeding
Hydra , jellyfish and corals show tissue-level organisation. Tentacles capture prey, but a single opening serves both for food intake and for waste removal.
Platyhelminthes (flatworms) — Bilateral symmetry and directional movement
Flatworms show bilateral symmetry , with distinct head-tail and front-back regions. Their flattened bodies allow gas exchange without respiratory organs, but there is still a single opening. Many are parasites with hooks and suckers.
Nematoda (roundworms) — Efficient body design with two openings
Roundworms have elongated, cylindrical bodies with two openings (mouth and anus), and show organ system level organisation distinct in male and female worms.
Annelida (segmented worms) — segmentation and body cavities
Earthworms have cylindrical bodies divided into segments , with a body cavity. Segmentation allows greater flexibility and more precise control of movement.
Arthropoda — Jointed appendages and an external skeleton
Insects, crabs and spiders have segmented bodies with jointed appendages and a hard exoskeleton that protects the body, reduces water loss and supports powerful muscles.
Mollusca — Organ system level organisation with soft bodies
Snails, squids and octopuses have soft bodies with organ system level organisation, often protected by a shell, with a distinct head, a muscular foot and a hump.
Echinodermata — Internal support without a notochord
Starfish and sea urchins have a hard internal skeleton of calcium carbonate . They lack a notochord, yet this endoskeleton gives protection and controlled movement.
NCERT Question 1 — Meena and Hari observed an
| Phylum (example) | Habitat | Level of organisation | Skeleton |
|---|---|---|---|
| Porifera (sponge) | Water (marine) | Cellular | ✗ |
| Cnidaria (jellyfish) | Water (fresh and marine) | Tissue | ✗ |
| Platyhelminthes (flatworm) | Water / inside host | Organ | ✗ |
| Nematoda (roundworm) | Soil / water / inside host | Organ system (digestive system) | ✗ |
| Annelida (earthworm) | Moist soil / water | Organ system | ✗ |
| Arthropoda (spider) | Land / water | Organ system | Exoskeleton |
| Mollusca (snail) | Water / moist land | Organ system | Exoskeleton |
| Echinodermata (starfish) | Marine water | Organ system | Endoskeleton |
Research studies show that one kilogram of sponge can filter up to 24,000 litres of sea water per day — drawing in food and oxygen through the countless pores in its body.
Some infectious diseases are caused by parasitic worms that enter through contaminated water and food and live in our alimentary canal, taking nutrients from our body (the organism they live on is the host ). To stay safe, wash hands properly, maintain personal hygiene, eat properly cooked food and drink boiled or filtered water.
- Animalia — the kingdom of multicellular, heterotrophic eukaryotes that depend on other organisms for food.
- Notochord — a flexible rod-shaped structure; in some chordates it is a precursor of the vertebral column.
- Chordata — animals that possess a notochord; further classified as protochordata and vertebrata.
- Non-chordata (Invertebrata) — animals that lack a notochord.
