Class XI · First Year · Sindh / BIEK · Chapter 11

Kingdom Animalia.

Animals are multicellular eukaryotes that eat other organisms, usually move, and respond quickly to their world. From sponges to humans, they are sorted into phyla by their body plan.

1 · What makes an animal

Members of Animalia are multicellular, eukaryotic heterotrophs that feed by ingestion (holozoic — taking food in and digesting it internally). Their cells have no cell wall and no chloroplasts. Most can move and have nerves and muscles for fast responses.

2 · How animals are grouped

Animals are sorted into phyla using features of the body plan:

Symmetry — none (sponges), radial (round, like a wheel — jellyfish) or bilateral (left = right mirror — most animals).
Body layers, presence of a body cavity, and segmentation.
Above all, whether they have a backbone: invertebrates (no backbone — about 97% of animals) vs vertebrates (with a backbone).

3 · The major phyla

Phylum	Examples	Key feature
Porifera	Sponges	No symmetry, no true tissues; body full of pores
Cnidaria	Jellyfish, Hydra, corals	Radial symmetry; stinging cells
Platyhelminthes	Flatworms, tapeworm	Flat, bilateral; some parasitic
Nematoda	Roundworms	Cylindrical, pointed ends; many parasitic
Annelida	Earthworm, leech	Body of repeating segments
Arthropoda	Insects, crabs, spiders	Jointed legs + chitin exoskeleton; largest phylum
Mollusca	Snails, octopus	Soft body, often a shell; muscular foot
Echinodermata	Starfish, sea urchin	Spiny skin; radial (adult); water vascular system
Chordata	Fish → mammals	Notochord & nerve cord; includes the vertebrates

4 · The vertebrates

Within the chordates, the vertebrates have a backbone and are grouped into five classes: fish (gills, fins), amphibians (moist skin, water + land — frogs), reptiles (dry scaly skin, shelled eggs), birds (feathers, wings) and mammals (hair, mammary glands; e.g. humans).

Invertebrates rule by numbers Although vertebrates are the most familiar animals, over 95% of all animal species are invertebrates — and the arthropods (especially insects) alone outnumber all other animals combined.

In one minute

Animals = multicellular heterotrophs; no cell wall, ingest food, usually move, have nerves & muscles.
Grouped by body plan: symmetry (none / radial / bilateral) and backbone (invertebrate vs vertebrate).
Major phyla: Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata, Chordata.
Vertebrate classes: fish, amphibians, reptiles, birds, mammals.

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