Lesson 1 of 12 · 7 min

Levels of organisation

NCERT §4.1, §4.1.1

On day one Ammu's notebook is a mess: a sponge from the tide pool, a jellyfish, a flatworm under a stone, an earthworm and a crab. They look nothing alike. Her teacher says the trick is to stop looking at shape and start asking how the body is put together.

The story this chapter follows: Ammu's harbour-to-backyard notebook

Ammu lives in a coastal town. Her biology teacher asks the class to sort every animal they meet in one week into its phylum and class. She starts at dawn at the harbour fish market, with prawns, crabs, squid, cuttlefish, oysters, a small shark, a sting ray, rohu, katla and magur on the ice. She walks the tide pools at low tide, then comes home to a monsoon backyard full of earthworms, frogs, snails, mosquitoes, a garden lizard, crows and pigeons.

The lesson in notes

In short

Animals are grouped by features they share in spite of differences in form: how cells are arranged, body symmetry, the kind of coelom, and the pattern of digestive, circulatory and reproductive systems.

Every animal is multicellular, but cells are organised at different levels. Sponges have the cellular level: cells form loose aggregates with some division of labour among them.

Coelenterates have the tissue level: cells doing the same job are grouped into tissues.

Platyhelminthes show the organ level, where tissues are grouped into organs, each specialised for one function.

Annelids, arthropods, molluscs, echinoderms and chordates show the organ-system level: organs work together as systems, each serving one physiological function.

An incomplete digestive system has a single opening that acts as both mouth and anus, as in Platyhelminthes. A complete one has two openings, a mouth and an anus.

In an open circulatory system the heart pumps blood out so that it bathes cells and tissues directly. In a closed system blood stays inside vessels of different diameters: arteries, veins and capillaries.